When trying to change an artist relationship from (founding) member to collaboration, I received a traceback, which I've attached. It seems easily reproduceable (happened twice to me).

Astute maintainers may have already spotted the issue - the (founding) flag is valid on a member relationship but not on the collaboration relationship. When I unchecked the attribute before changing its type, the edit was accepted. Clearly, good user interface practice would be to check for this rather than throw an exception, though.

Here's a link to the successful edit https://musicbrainz.org/edit/30764943