Farmer claims "KeyError" but the keys are correct

in my logfile I get an error every time the farmer finds a proof.
The error-message is:
WARNING KeyError plot [plotname].plot does not exist.

But, the plotfile exists and the farmer- and poolkey are also correct, the plotfiles have the same key as the keys in the software.

What is wrong here? I have no idea where to look for the cause of this error.

Platform: ubuntu 20.04
The same error occurs in chia and every chia-fork that I have tried so far.
The plots were made with “madmax”

THX, stn