This may be caused by a communication error with the database server. The system may have been able to mostly recover from the error, but the knowledge index cache may get out of sync.
1. Stop the Service Desk services completely
2. Go into the Service Desk install folder, then open the "site" folder. There should be two folders in there, "ebr" and "ebr_ADM", remove these two folders from the directory
3. Start Service Desk services, and run the pdm_k_reindex command, it may take a bit of time to completely rebuild the index depending on the number of documents in the environment