Can you open your iDisk in Finder?
My wife's iMac had a similar problem to what you're seeing. The problem showed up in Finder and Backup; I didn't try OmniFocus. The root cause was that I had entered Lisa's email-only .mac address when installing Leopard. Despite the fact that I entered my full-featured .mac address in System Preferences --> .Mac, the OS was still trying to attach to the iDisk using her address. Ultimately I had to:
- Sign-out of the .Mac account in System Preferences
- Log-out of her account on the Mac
- Log back in
- Reconfigure System Preferences --> .Mac to use my account
I may have done one more cycle of logging out and in to the Mac at the end. It was really frustrating trying to figure out where the OS was caching the .Mac connection information. I tried lots of things. The list above is what I recall as the last steps before it started working. (I also may have walked down to the fridge for a Diet Coke, but that probably didn't effect the .Mac configuration.)
If you can recreate the problem just using Finder to connect to your iDisk, then that will simplify troubleshooting.
Good luck, and let us know how it goes.