Yesterday I created an on-demand real time workflow that resolves a case if there are no tasks active and stops workflow with a status message if there are any tasks still active, it worked perfectly and the status message was displayed to the user without any problems.
Today for some reason it just won't display that popup message anymore, I tried deactivating and reactivating the workflow, checked the process session and it fails just as expected and the status message is shown in the stack trace.