portainer: Can't use external template : Unable to parse external templates on fresh Portainer install

I’m trying to use the external template functionality to use my versioned templates however I receive an “Unable to parse external templates” error (even with the raw official source for templates.json). The issue is not similar to #2283, because it happens on a fresh Portainer install.

Steps to reproduce the behavior:

1. Fresh install Portainer 1.19.2
2. Activate external template in settings
3. Put the official template.json url (raw) : https://raw.githubusercontent.com/portainer/templates/master/templates.json
4. Go to Template : the error should appear

Technical details:

• Portainer version: 1.19.2 (fresh install)

Client:
 Version:           18.06.1-ce
 API version:       1.38
 Go version:        go1.10.3
 Git commit:        e68fc7a
 Built:             Tue Aug 21 17:24:51 2018
 OS/Arch:           linux/amd64
 Experimental:      false

Server:
 Engine:
  Version:          18.06.1-ce
  API version:      1.38 (minimum version 1.12)
  Go version:       go1.10.3
  Git commit:       e68fc7a
  Built:            Tue Aug 21 17:23:15 2018
  OS/Arch:          linux/amd64
  Experimental:     false