ipympl: "Error displaying widget: model not found" again

After an update of Jupyterlab I cannot use ipympl anymore as the minimal example

%matplotlib widget
from matplotlib import pyplot as plt

fig, ax = plt.subplots()

only shows “Error displaying widget: model not found”.

Unfortunately something in the Jupyter/matplotlib/ipympl machinery seems very fragile so that this is easily triggered. In the past a simple re-install of ipympl helped but this does not seem to work here at the moment.

My environment looks like

 3.8.10 | packaged by conda-forge | (default, May 11 2021, 06:25:23) [MSC v.1916 64 bit (AMD64)]
ipympl version: 0.7.0
jupyter core     : 4.7.1
jupyter-notebook : 6.4.0
qtconsole        : not installed
ipython          : 7.23.1
ipykernel        : 5.5.5
jupyter client   : 6.1.12
jupyter lab      : 3.0.15
nbconvert        : 6.0.7
ipywidgets       : 7.6.3
nbformat         : 5.1.3
traitlets        : 5.0.5
Config option `kernel_spec_manager_class` not recognized by `ListNBExtensionsApp`.
Known nbextensions:
  config dir: C:\tools\miniconda3\envs\jupyter\etc\jupyter\nbconfig
    notebook section
      jupyter-matplotlib/extension enabled
      - Validating: ok
      jupytext/index enabled
      - Validating: ok
      nbdime/index enabled
      - Validating: ok
      voila/extension enabled
      - Validating: ok
      jupyter-js-widgets/extension enabled
      - Validating: ok
Config option `kernel_spec_manager_class` not recognized by `ListLabExtensionsApp`.
[W 2021-05-20 18:11:49.703 LabApp] Config option `kernel_spec_manager_class` not recognized by `LabApp`.
JupyterLab v3.0.15
C:\tools\miniconda3\envs\jupyter\share\jupyter\labextensions
        jupyter-matplotlib v0.9.0 enabled ok
        jupyterlab-jupytext v1.3.1 enabled ok (python, jupytext)
        nbdime-jupyterlab v2.1.0 enabled ok
        @jupyter-widgets/jupyterlab-manager v3.0.0 enabled ok (python, jupyterlab_widgets)
        @ryantam626/jupyterlab_code_formatter v1.4.10 enabled ok (python, jupyterlab-code-formatter)
        @voila-dashboards/jupyterlab-preview v2.0.2 enabled ok (python, voila)

Chrome complains in the Javascript console that Uncaught (in promise) Error: Module jupyter-matplotlib, semver range ^0.9.0 is not registered as a widget module and earlier errors such as Plugin 'matplotlib-jupyter:main' failed to activate.

When using a completely fresh conda environment ipympl has always worked here, but something breaks when updating parts of the environment and then I do not see what is broken and an simple reinstall (conda uninstall ipympl, conda install -c conda-forge ipympl) works but the error message remains and the plot cannot be displayed.

Any hint how to diagnose or prevent this from happening would be appreciated.