Sorry for the trouble here, Martin! This often means that more than one user name or password for the account you're syncing with has been saved to the Keychain on your Mac. In cases like you describe, OmniFocus is grabbing one of the incorrect copies, causing the server to kick the password back at us.
If you use the "Keychain Access" application in your Utilities folder to examine the contents of your keychain, you may be able to find any duplicate or incorrect entries. After removing them, restart OmniFocus and try to sync. If that doesn't resolve the problem, email the
support ninjas and we'll be happy to investigate this with you. Hope this helps!