Create folder content type: error "SPFileCollection.Add()"


I created a custom content type "Department" based on the "folder" content type.
I added a Plumsail form to this content type.
When I create a new "Department" in my document library, I get following error:

To add an item to a document library, use SPFileCollection.Add()

What am I doing wrong?

kind regards,

Dear @bartplessers,
This seems like a bug - reported to our dev team, they'll investigate the issue and see if it can be resolved as quickly as possible. Thank you for the report!