[Brian]

NOTE: these steps were valid at the time they were posted, but as of 
the time of this writing (Nov. 2017) google is still presenting them for certain 
searches. They will not help customers using current releases of 
OmniFocus resolve this problem! Contact our Support Humans for help 
instead.

Posting the response we sent Leanda in case other folks run across this problem in the future.

If you're syncing OmniFocus with another Mac, or an iPhone/iPod, you can resolve the issue by doing the following on the machine that's presenting the error. If you're not syncing, we can still help; just skip down to the bottom of this post.

To resolve the problem on a machine that's being synced, do the following:

1. Quit OmniFocus.
2. Select "Home" from the "Go" menu in the Finder. A new window will appear.
3. Starting in the Home folder selected in the new window, open "Library", then "Application Support", and finally the "OmniFocus" folder.
4. You should see a file named "OmniFocus" or "OmniFocus.ofocus". Please rename it to "OLD-OmniFocus.ofocus".
5. Launch OmniFocus; it should open the blank "welcome" database.
6. Go ahead and sync OmniFocus. When it asks you to pick the "local" or "server" copy of the database, choose Server.
7. Once that sync goes through, go ahead and relaunch OmniFocus.

If OmniFocus launches without error, you should be all set. We have plans to add code that does all this automatically, but it's not in place yet - apologies for the inconvenience this caused.

If you're not syncing, or if that didn't help, please contact the support ninjas after doing the following.

1. Select Go -> Home from the menu bar in the Finder.
2. Switch to the "Documents" folder.
3. Open the "OmniFocus Backups" folder.
4. Double-click the most recently-created backup in that folder. If it presents the same error message, close the window and repeat with the previous backup. Keep opening backups until you reach one that opens without any error messages.
5. Send the backup you opened and your "real" OmniFocus database to the support ninjas. We'll repair the problem. Your database is located here:
   <your home folder>/Library/Application Support/OmniFocus

We'll use the files to get things sorted out ASAP. Apologies for the trouble.