FILES do not work

I am using two instances of Microsoft Teams: one is a school account in the University I am working in which is connected to my University ID.
The other is a Free Version I am using for my private business. To login I use a Microsoft ID with an Email address of my own URL. I am also using the same ID für a Office Business Family Service.

In the latter accounts FILES functionality doesn't work and therefore also other services related to files-functionality, like WIKI ...
The Error Message I get is «Your Files cannot be accesses - we are working to restore them» (sorry for the translation from german ...) but that never happens.
For the original Error messages see the attached images.

What is maybe interesting to mention: some colleagues of mine have a similar setup and for them FILES are working in some, but not in other instances of their teams.