Hi Andrezej
Have a look at the following post for this issue. It may be that you need to complete the workflow or cancel it to remove the lock. There was a note some time last year pertaining to this error and BRM.
AC 10- BRM Unable to delete Role in Stucked Phase: Request Approval Pending
I would not recommend updating SAP standard table directly without direction by SAP
Double-check the WF via SWIA is actually closed out as well. Not sure what SP you are on but this was an issue last year and SAP released a note for it. I think that's referenced in the post above.
Regards
Colleen