An error occurred while rendering the editor

I’m a teacher and I’m using gitlab with my students to do some simple stuff like html and css. We have our own instance on a self-hosted server. (My husband does the admin-stuff.)

On my computer and on three devices used by my pupils the above error occurs when trying to edit something in our first dokument (which is a index.html): "An error occurred while rendering the editor " The editor would remain blank as seen in the picture.

The error does show up using Windows 10, Browser is Firefox. It also showed with the same Windows-User using Chrome, but surprisngly not on the same computer with another Windows-User.
I installed Chromium instead of Chrome and was surprised again - no error showed up on both Windows-Users.
Some gitlab-user on another PC oder even a smartphone did not have any problems.

I searched online but didn’t find any clues. I reset Chrome to default settings because I assumed it could be some kind of blocker or plugins I used, but the error stayed the same.
Can you help me? It’s no problem for me to use Chromium, but I’d love to give my students a hint, why it’s not working on their devices (but on all of the classmates…)

Thanks a lot and stay healthy!