3453231 - Group Reporting: Task already blocked System Error in Manage Data Validation Tasks app

In the Manage Data Validation Tasks app, you get a System Error, when you run a validation task in update mode.
If you click to the "View Log" button, you get a message, that "Task & is already blocked in period &, version & for CG/CU &", although the task is not blocked in the Data Monitor/Consolidation Monitor.

SAP S/4HANA Finance for Group Reporting 

Group Reporting, Validation, Validation Engine, Manage Data Validation Tasks, Lock entry, Obsolete lock entry, VECAM, Task is already blocked, System Error, update mode, test mode , KBA , FIN-CS-VE , Validation Engine , Problem