The first thing to do here would be to make sure that both OmniFocus for Mac and OmniFocus for iPhone are fully updated. (So you want 1.7.5 on the Mac, and 1.5.3 on the iPhone.)
There was a problem that could cause that error message in earlier versions of the app, but that issue has been fixed in the versions I mentioned above.
I just got back from a couple of weeks away from the office; just in case there's something more recent that I don't know about, contact the
support ninjas if you're fully up to date and are still seeing that message.