No real solution, but you could try setting the synchronization and connection bus loggers to trace.
Mendix 7 is more restrictive on entity access. Maybe you create an object offline in such a way that the user has no access to it according to domain model access.
Or you attempt to upload a new image where the user has insufficient access rights.
These loggers usually have some info about this. At least you will find the last SQL statement before it failed, should help point you in the right direction.