Error message "RequestValidationFailed" when attempting to modify a copy job by the owner

Hi SimoneZier ,

Thank you for reaching out to Microsoft Fabric Community.

Thank you tayloramy  for the prompt response.

This is an issue with Fabric Copy Jobs: even after you “Take Over” a job, the system still keeps the original creator tied to the job internally. Because of that, certain changes-like switching from append to overwrite-can only be made by the person who originally created the job.
That’s why your new jobs (where you are the creator) work, but taken-over jobs fail with RequestValidationFailed.
The only reliable workaround today is to recreate the copy job under your own account. Microsoft is aware of this limitation and a fix is expected in a future update.