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

In the Manage Data Validation Tasks app (App ID: F2653), you get a System Error, when you run a validation task in update mode (like the Reported Data Validation, the Standardized Data Validation or the Consolidated Data Validation tasks).
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, F2653 , KBA , FIN-CS-VE , Validation Engine , Problem