Run a Data Consistency Check.
If there is nothing to post in your Data Consistency check, then It seems your DB indices have become corrupt. To fix this you need to run a database optimisation, which is basically a backup, followed by a restore. To run a database optimisation, everyone must remain logged out. Database optimisation is run from Tools | Database Maintenance | Database Optimisation in the Omni Control Centre. Please let us know if you experience this problem, so we can keep tabs on how prolific it is.