I just want to add that I already cleaned the cache without success on fixing this.
I also used the Firefox web console to see if there were any errors and could only get this error once:
I also used the Firefox web console to see if there were any errors and could only get this error once:
I reproduced the problem a few times and couldn't get the same error. In fact Firefox doesn't detect any errors, it seems.
I'm not familiar with Moodle debugging tools, so I haven't used them yet, but another administrator is checking that now.
OBS (in case it is relevant): Earlier in the day my complete site was down due to an error while connecting with the server. We contacted our server provider and they said it was due to over quota and it was fixed. But then I noticed this error I'm reporting here.