Add a Card - Denied fields combination detected (Open Unassigned)

Hello,

So I have the following setup. A project that was scrum and I'd like to move it to kanban. Now I deattached the scrum workflow. Created a new agile board with kanban style and added a swimlane like the following.

Now I do get all my stories split nicely, but when I try to create a new card I gett the following error message

Denied fields combination detected (Open Unassigned)

Some more information:

The project was imported from JIRA