This message means that OmniFocus is able to communicate with your sync server, but the server is not letting you access the location the app is trying to work with.
The most common cause of this message is the server’s configuration has changed, but in a way that doesn’t tell OmniFocus that it needs to ask you for updated login information.
OmniFocus attempts to work around this by prompting you for updated credentials in most cases, but if you do see this message you can try temporarily changing your password on the server. This will cause it to send OmniFocus a different error message which the app will recognize as meaning that we need to prompt for new login info. (Another option is to try telling OmniFocus to sync to me.com if it’s currently configured to sync to one of the older mac.com addresses.)
A good next step is to make sure that you have plenty of space left on your iDisk or WebDAV server. For most people, a few megabytes is enough, but if you have lots of attachments or an especially large database, you may need more space.
If that doesn’t fix the problem, the next step is to check the document path specified in sync preferences. If it is incomplete or misspelled, this error may occur.
Another thing to try would be to check the permissions set on the folder or account you’re syncing with on the server. Certain WebDAV accounts especially may be configured incorrectly, or may be configured for read-only access from some accounts.