This is generally caused by @DbLookup/@DbColumn on a view and the lookup key is not found.
However, it can also be caused by a save/replication conflict in the same view used for the lookups. If this is true, you can add "(!@IsAvailable($Conflict))" to the view selection formula to exclude conflicts.
It can also be caused by the view setting "Generate Unique Keys in Index for ODBC" if there's a replication conflict in the same view. In this case, you'll get an error if you try to open the view but you don't see any formulas or events that are doing any lookups.