Thank you Andy. In my case the projects are in the Recycle bin and I can neither delete them nor restore them. Unfortunately I just tried that action again and with the same result.
Task 557614516 failed: Unhandled Exception: Child Task 557614520 error: Unhandled Exception: {"message":"VolumeDriver.Mount: Error creating mount record for volume='d4b3ca6f-a8d7-4127-804f-e99f68a6c6b2': HTTP 400: Unable to mount