First, you need to get your IDB backed up. In the Backup specification for your Internal Database, go to the Backup Object Summary tab. Right-click in the [Database] object, and pick Properties, or just click the Properties button
Go to the Internal Database tab, and un-select "Check Internal Database". Click OK and Apply. This will enable you to continue backing up the IDB while you get teh 'omnidbcheck' problem sorted out
You can take the parts of the 'omnidbcheck -quick' or 'omnidbcheck -extended' and run them individually
omnidbcheck -dc
omnidbcheck -bf
omnidbcheck -filenames
omnidbcheck -core
to see if there is any corruption or inconsistency being detected
I think if you waiting long enough, several hours perhaps, the 'core' part would come back with something, since the IDB is 17GB. But, how long since you did a purge of your IDB?
omnidbutil -purge -filenames -force
omnidbutil -writedb ..........
omnidbutil -readdb ..........
If it has been quite a long time, or, if you never remember doing it at all, I think it needs to be done