If it's still complaining about the exact same file path, you probably have an empty folder on your iDisk named "OmniFocus.ofocus".
If you're not seeing a folder or file with that name in the documents folder of your iDisk, then something else is going on here. (Do you have Apple's iDisk syncing turned on? It's been known to cause problems - more details are available
here.)
In any case, the thing to do here is probably send the exact text of the error message you're getting to the Support Ninjas using Send Feedback under the Help menu; they'll be happy to help you get this straightened out.