Hi JorgeB,
Thank you for your reply!
In the mean time I moved a lot of data from disk4 to disk2 and now it seems to work again...
But usually an operation like you can see in the second screenshot would not work before. And it would give me the error posted above.
I just tried out an even "bigger" move, where I moved two complete tv shows from one share to another and it also worked.
But is there a general problem to fill up disks in Unraid? (There is even an option for the shares to fill up disks)