Our user with the GUID {15b0e4a8-9819-42bd-9fb5-0f143e3a8aa1} was unable to assign the task with the GUID {36aa12f2-b8f9-4c8d-8ade-bf66b50bd564} to themselves. The php_errors.log showed a database integrity constraint violation.

It turned out there was still an entry in LEK_taskUserAssoc with the state "edit" although she had already left the task. She logged out and back in multiple times but that did not help. I have now removed that line manually and attached a csv export of that user's rows from LEK_taskUserAssoc.