This is the Problem number.   PRB625314.   If you cannot see it in HI, then maybe you can refer to it if you have to call support.   Here is the description:

'Invalid group change' message is displayed when workflows assigns group to HR task:

This is issue is caused by   the sm_task's ValidateChanges BR preventing workflows to set an assignment group to an HR Task.

Furthermore, the ValidateChanges BR cannot be deactivated by admin: its policy is set to 'read'