I'm also getting the message "Only the default document can be synchronized".
I have tried...
- resetting sync history in isync
- completely deleting the Omnifocus data files in ~Library/Application Support/Omnifocus and restarting the program
- importing and exporting the data file
- completely "AppZapping" the program and reinstalling
All to no avail.
This is particularly painful to me since I live on my Palm which syncs with iCal, and now I don't have any tasks at all.
Help me, Support Ninjas, you're my only hope!