(@mods
Please feel free to move the last entries to a new topic....)

@jPV

You are absolutely right - and I had the same idea :) But no luck :(

I can access the system partition without any problems. And yes, I tried to find dirs or files with a large size. But I did not.

I guess something went wrong because of the window which appeared saying: "Error while checking the system. Block 10990838 invalid identifier"

This, by the way, also comes, if I boot via USB stick during the selection of language and keyboard.

I do not know if this partition has a different filesystem because I bought it ready-to-use. I will try the info command, suggested by you. And I will also ask the retailer if they can tell me.

Thanks so far!