mkdocs-material: yaml.constructor.ConstructorError: could not determine a constructor for the tag

Starting today I’m getting the following error during a build, same for python 2 and 3.

https://app.wercker.com/Wodby/php-docs/runs/build/5b3365e4a296ce0001c90842?step=5b3365ec37b4370001c4f556

Collecting mkdocs-material
  Downloading https://files.pythonhosted.org/packages/38/3c/f22621b0d5b349e7ee6b711255ebdf02425ebd775267a0e959947eb40325/mkdocs_material-2.9.1-py2.py3-none-any.whl (570kB)
Collecting markdown-include
  Downloading https://files.pythonhosted.org/packages/ef/44/eb6e9b4fa1110b719abb876c9b6dd8b46af886a94536ec4e9117fe5e7b97/markdown-include-0.5.1.tar.gz
Collecting pygments
  Downloading https://files.pythonhosted.org/packages/02/ee/b6e02dc6529e82b75bb06823ff7d005b141037cb1416b10c6f00fc419dca/Pygments-2.2.0-py2.py3-none-any.whl (841kB)
Collecting pymdown-extensions>=4.10 (from mkdocs-material)
  Downloading https://files.pythonhosted.org/packages/7d/0b/8a645733fc6ad20c1217808d2bf26eb80996901d173e623f886b75f62239/pymdown_extensions-4.11-py2.py3-none-any.whl (205kB)
Collecting tornado<5 (from mkdocs-material)
  Downloading https://files.pythonhosted.org/packages/e3/7b/e29ab3d51c8df66922fea216e2bddfcb6430fb29620e5165b16a216e0d3c/tornado-4.5.3.tar.gz (484kB)
Collecting mkdocs>=0.17.1 (from mkdocs-material)
  Downloading https://files.pythonhosted.org/packages/66/05/5a9e9f661bfd554950d71ead196231f15fe744bf7d1d2431ee1daa32841d/mkdocs-0.17.4-py2.py3-none-any.whl (1.2MB)
Collecting markdown (from markdown-include)
  Downloading https://files.pythonhosted.org/packages/6d/7d/488b90f470b96531a3f5788cf12a93332f543dbab13c423a5e7ce96a0493/Markdown-2.6.11-py2.py3-none-any.whl (78kB)
Collecting singledispatch (from tornado<5->mkdocs-material)
  Downloading https://files.pythonhosted.org/packages/c5/10/369f50bcd4621b263927b0a1519987a04383d4a98fb10438042ad410cf88/singledispatch-3.4.0.3-py2.py3-none-any.whl
Collecting certifi (from tornado<5->mkdocs-material)
  Downloading https://files.pythonhosted.org/packages/7c/e6/92ad559b7192d846975fc916b65f667c7b8c3a32bea7372340bfe9a15fa5/certifi-2018.4.16-py2.py3-none-any.whl (150kB)
Collecting backports_abc>=0.4 (from tornado<5->mkdocs-material)
  Downloading https://files.pythonhosted.org/packages/7d/56/6f3ac1b816d0cd8994e83d0c4e55bc64567532f7dc543378bd87f81cebc7/backports_abc-0.5-py2.py3-none-any.whl
Collecting livereload>=2.5.1 (from mkdocs>=0.17.1->mkdocs-material)
  Downloading https://files.pythonhosted.org/packages/dd/b4/213daced3ff1b4e02a1f700748e20e9a7481f5bfef57d11ae9babfd4aa2f/livereload-2.5.2-py2.py3-none-any.whl
Collecting PyYAML>=3.10 (from mkdocs>=0.17.1->mkdocs-material)
  Downloading https://files.pythonhosted.org/packages/10/7d/6efe0bd69580fecd40adf47ebaf8d807238308ccb851f0549881fa7605aa/PyYAML-4.1.tar.gz (153kB)
Collecting click>=3.3 (from mkdocs>=0.17.1->mkdocs-material)
  Downloading https://files.pythonhosted.org/packages/34/c1/8806f99713ddb993c5366c362b2f908f18269f8d792aff1abfd700775a77/click-6.7-py2.py3-none-any.whl (71kB)
Collecting Jinja2>=2.7.1 (from mkdocs>=0.17.1->mkdocs-material)
  Downloading https://files.pythonhosted.org/packages/7f/ff/ae64bacdfc95f27a016a7bed8e8686763ba4d277a78ca76f32659220a731/Jinja2-2.10-py2.py3-none-any.whl (126kB)
Collecting six (from singledispatch->tornado<5->mkdocs-material)
  Downloading https://files.pythonhosted.org/packages/67/4b/141a581104b1f6397bfa78ac9d43d8ad29a7ca43ea90a2d863fe3056e86a/six-1.11.0-py2.py3-none-any.whl
Collecting MarkupSafe>=0.23 (from Jinja2>=2.7.1->mkdocs>=0.17.1->mkdocs-material)
  Downloading https://files.pythonhosted.org/packages/4d/de/32d741db316d8fdb7680822dd37001ef7a448255de9699ab4bfcbdf4172b/MarkupSafe-1.0.tar.gz
Building wheels for collected packages: markdown-include, tornado, PyYAML, MarkupSafe
  Running setup.py bdist_wheel for markdown-include: started
  Running setup.py bdist_wheel for markdown-include: finished with status 'done'
  Stored in directory: /root/.cache/pip/wheels/af/4c/e3/599182bf5f723d76154ecf793632fc05f65250ba55a4a5bdce
  Running setup.py bdist_wheel for tornado: started
  Running setup.py bdist_wheel for tornado: finished with status 'done'
  Stored in directory: /root/.cache/pip/wheels/72/bf/f4/b68fa69596986881b397b18ff2b9af5f8181233aadcc9f76fd
  Running setup.py bdist_wheel for PyYAML: started
  Running setup.py bdist_wheel for PyYAML: finished with status 'done'
  Stored in directory: /root/.cache/pip/wheels/57/8e/30/4ef5a500e763c11371ee16ec7f68043efc832ed1ff0533adff
  Running setup.py bdist_wheel for MarkupSafe: started
  Running setup.py bdist_wheel for MarkupSafe: finished with status 'done'
  Stored in directory: /root/.cache/pip/wheels/33/56/20/ebe49a5c612fffe1c5a632146b16596f9e64676768661e4e46
Successfully built markdown-include tornado PyYAML MarkupSafe
Installing collected packages: markdown, pymdown-extensions, six, singledispatch, certifi, backports-abc, tornado, pygments, livereload, PyYAML, click, MarkupSafe, Jinja2, mkdocs, mkdocs-material, markdown-include
Successfully installed Jinja2-2.10 MarkupSafe-1.0 PyYAML-4.1 backports-abc-0.5 certifi-2018.4.16 click-6.7 livereload-2.5.2 markdown-2.6.11 markdown-include-0.5.1 mkdocs-0.17.4 mkdocs-material-2.9.1 pygments-2.2.0 pymdown-extensions-4.11 singledispatch-3.4.0.3 six-1.11.0 tornado-4.5.3
Traceback (most recent call last):
  File "/usr/local/bin/mkdocs", line 11, in <module>
    sys.exit(cli())
  File "/usr/local/lib/python2.7/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python2.7/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python2.7/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python2.7/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python2.7/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python2.7/site-packages/mkdocs/__main__.py", line 155, in build_command
    site_dir=site_dir
  File "/usr/local/lib/python2.7/site-packages/mkdocs/config/base.py", line 177, in load_config
    cfg.load_file(config_file)
  File "/usr/local/lib/python2.7/site-packages/mkdocs/config/base.py", line 123, in load_file
    return self.load_dict(utils.yaml_load(config_file))
  File "/usr/local/lib/python2.7/site-packages/mkdocs/utils/__init__.py", line 71, in yaml_load
    return yaml.load(source, Loader)
  File "/usr/local/lib/python2.7/site-packages/yaml/__init__.py", line 74, in load
    return loader.get_single_data()
  File "/usr/local/lib/python2.7/site-packages/yaml/constructor.py", line 39, in get_single_data
    return self.construct_document(node)
  File "/usr/local/lib/python2.7/site-packages/yaml/constructor.py", line 48, in construct_document
    for dummy in generator:
  File "/usr/local/lib/python2.7/site-packages/yaml/constructor.py", line 398, in construct_yaml_map
    value = self.construct_mapping(node)
  File "/usr/local/lib/python2.7/site-packages/yaml/constructor.py", line 208, in construct_mapping
    return BaseConstructor.construct_mapping(self, node, deep=deep)
  File "/usr/local/lib/python2.7/site-packages/yaml/constructor.py", line 133, in construct_mapping
    value = self.construct_object(value_node, deep=deep)
  File "/usr/local/lib/python2.7/site-packages/yaml/constructor.py", line 88, in construct_object
    data = constructor(self, node)
  File "/usr/local/lib/python2.7/site-packages/yaml/constructor.py", line 414, in construct_undefined
    node.start_mark)
yaml.constructor.ConstructorError: could not determine a constructor for the tag 'tag:yaml.org,2002:python/name:pymdownx.emoji.to_svg'
  in "/pipeline/source/mkdocs.yml", line 49, column 24

My build command:

pip install mkdocs-material markdown-include pygments
mkdocs build

mkdocs.yml:

site_name: PHP stack documentation
site_description: PHP stack documentation
site_author: Wodby, Inc.
site_url: https://wodby.com/stacks/php/docs

repo_name: wodby/php-docs
repo_url: https://github.com/wodby/php-docs

copyright: Wodby, Inc.

extra:
  social:
    - type: globe
      link: https://wodby.com
    - type: github-alt
      link: https://github.com/wodby
    - type: twitter
      link: https://twitter.com/WodbyHQ
  stack_url: https://cloud.wodby.com/stackhub

theme:
  name: material
  feature:
    tabs: true
  palette:
    primary: 'indigo'
    accent: 'indigo'
  custom_dir: 'theme'

extra_css: ["assets/wodby.css"]
extra_javascript: ["assets/intercom.js"]

markdown_extensions:
  - markdown.extensions.admonition
  - markdown.extensions.codehilite:
      guess_lang: false
  - markdown.extensions.def_list
  - markdown.extensions.footnotes
  - markdown.extensions.meta
  - markdown.extensions.toc:
      permalink: true
  - pymdownx.arithmatex
  - pymdownx.betterem:
      smart_enable: all
  - pymdownx.caret
  - pymdownx.critic
  - pymdownx.details
  - pymdownx.emoji:
      emoji_generator: !!python/name:pymdownx.emoji.to_svg
  - pymdownx.inlinehilite
  - pymdownx.magiclink
  - pymdownx.mark
  - pymdownx.smartsymbols
  - pymdownx.superfences
  - pymdownx.tasklist:
      custom_checkbox: true
  - pymdownx.tilde

pages:
    - Home: index.md
    - Backups: backups.md
    - Deployment: deployment.md
    - Import: import.md
    - Instance types: instance-types.md
    - Mail delivery: mail-delivery.md
    - Containers:
      - Overview: containers/index.md
      - Apache: containers/apache.md
      - AthenaPDF: containers/athenapdf.md
      - Blackfire: containers/blackfire.md
      - Cron: containers/cron.md
      - Elasticsearch: containers/elasticsearch.md
      - Kibana: containers/kibana.md
      - Mailhog: containers/mailhog.md
      - MariaDB: containers/mariadb.md
      - Memcached: containers/memcached.md
      - Nginx: containers/nginx.md
      - OpenSMTPD: containers/opensmtpd.md
      - PHP: containers/php.md
      - PostgreSQL: containers/postgres.md
      - Rsyslog: containers/rsyslog.md
      - Redis: containers/redis.md
      - Solr: containers/solr.md
      - SSH: containers/ssh.md
      - Webgrind: containers/webgrind.md
    - Local environment:
      - Overview: local/index.md
      - Quick start: local/quick-start.md
      - Domains: local/domains.md
      - Multiple projects: local/multiple-projects.md
      - Permissions issues: local/permissions.md
      - Import and export: local/import-export.md
      - Make commands: local/make-commands.md
      - For macOS users: local/docker-for-mac.md
      - Xdebug: local/xdebug.md