Your Core Part of IDB got corrupted, by what I cannot say.
Re-run the omnidbcheck -core command and check what it complains about.
Also some details might be in Check_core.txt Log (/var/opt/omni/server/log on Linux/Unix)
If it is just a single session you may be able to repair the error by either deleting the corrupted Session/Objects and running a purge or by trying to re-import the Detail-Infos and overwrite the corrupted Information.