This error is most likely caused by an indexing problem with the Design Manager Database. Try running a Repair of the database?File ? Utilities ? repair the Database.
If that does not fix the issue, run the Live Update. If the problem persists after the ***Live Update, please contact Design Manager Technical Support as they may need to get a copy of your database to diagnose the problem.
***If you are a modified client, ALWAYS contact support before running a live update.***