Sorry for the trouble this is causing! Drew, we've gotten reports of a number of keychain-related issues from folks who've installed 10.8. Wok4, you don't mention which OS you have installed; if it's not Mountain Lion, let us know and we can investigate this with you.
Some customers seeing this have reported that using Keychain Access to remove the entry for sync.omnigroup.com is sufficient to get things back up and running normally, at least temporarily. (We've seen reports of similar keychain-related issues in other apps on Mountain Lion, so our current working theory is that this is something Apple's going to need to address.)
Another issue we've had reported on 10.8 is OmniFocus asking for your login information when the app launches in response to the clipping shortcut. (This doesn't appear to happen if OmniFocus is already running when you clip something.)
We're looking into this one, but for the time being, we'd suggest making OmniFocus one of your Login Items (via the "Users & Groups" pane of System Preferences), then hiding (rather than quitting) the app when you're not using it. That should reduce the number of times that panel appears.
If neither of those sound like what you're seeing, email our
support ninjas and we'll be happy to look into this with you. Sorry again for the trouble!