I'm sorry for the trouble this is causing! It sounds like something's wrong with the local database on that Mac.
Please quit OmniFocus, then open up
<your home folder>/Library/Application Support/OmniFocus
In that folder, you should see a file named "OmniFocus" or "OmniFocus.ofocus".
If you add "Old-" to the beginning of the name (naming it "Old-OmniFocus.ofocus" in other words) you should be able to launch the app.
If you're syncing, you should be able to pull a fresh copy of your data down from the server at that point.
If you're not syncing, you can choose "Revert to database backup" from the File menu, choose your most recent backup and get your data back that way.
Does that help? If not, contact the
support ninjas and we'll be happy to help some more. Apologies!