There's really no other answer. Go to the Sync preferences, delete both fields completely, restart Zotero for good measure, and then reenter them again. Make sure you're typing or copying completely, without leading/trailing spaces, etc.
If you're still having trouble, provide a Debug ID for a Sync attempt that gives you an error. (Your password won't be logged.)
If you're still having trouble, provide a Debug ID for a Sync attempt that gives you an error. (Your password won't be logged.)