Hi Paul,
Did you check out this thread? https://forum.mendix.com/questions/4370/Error-retrieving-records-in-datagrid--1-to-NaN-of-3
I came across the same nasty behaviour and found out the system is sometimes deleting items you do not expect. In my case, I think it was a rollback or delete on an orderline that removed the order as well. Something like that.
Cheers. Toon
Thanks for your answer Toon,
This helped me in finding that an assoc was deleted. The assignment ( object that needs to be displayed ) still exists, but one of it's assocs isn't set properly.
All i have to do is set the guid, and ofcourse find out why the assoc was deleted in the first place :-)