I received a similar message (see below) when using default save and cancel buttons.
In my case it was related to entity access. A reference was set automatically, by creating a new entity from a datagrad with association source, but the current user did not have write access to this reference.
Perhaps your problem is also related to access to references or attributes..
com.mendix.webui.WebUIException: No read only change hash found, while read only values are changed for object with id '24769797950537729'.
thanks. We found the issue. I added a record in entity A without commit and directly add AND commit a record in the linked Entity B ( some kind of logbook ) . That is wrong and Mx6 did not complain about this. Now the Entity B will be commit together with Entity A and everything is fine.