Finally got mine working, but I had to go a step further. After I deleted everything on my iDisk related to OmniFocus, I still got this message:

Unable to read document.

Old-format transaction file "root=14E791F6-6093-475E-8F5C-EDD95D013904:76D7D552-4607-43AD-B74A-CF92539F67A1.xml" must be upgraded before opening.


Then I deleted all _local_ backup files and finally it sync'd succesfully.