I've been in email-contact with you on this one, but posting it here in case anyone else wonders. A problem seems to occur when trying to move elements into a subfolder (or any other type I guess) of a deactivated folder. Error: "Relationfield not set or set to a deleted or unpublished item".
Hi, this was fixed in the latest build!:
WAF B1301 - AlphaCode
Added support for modifying relations on deavtivated contents with drag/drop operations in UI.