View the dependencies of the session. If anything is red, then that's the faulty asset. Sometimes, if nothing appears faulty, you will need to view dependencies recursively and then view dependencies of the dependencies - keep drilling down.
For built-in or DLC assets, you can try highlighting and choosing revert to original after right-clicking on the asset(s).
If this doesn't work, a DBR or even an EDBR will hopefully repair your database.