Error 503 when regenerating or deleting a watermark

Hello,

When I upload a new image and manually apply the watermark, everything is correct. But if I want to remove the watermark from the image or regenerate it (using apply again), I get an error – see screen. I have tried changing the template, deactivating other plugins, but the problem still occurs. Unfortunately, nothing is written to the log and thus the exact cause is not visible, but it seems strange to me that application would go, but e.g. deletion would not.

Thank you for your help.
Jan