In OS X 10.8 Mountain Lion and later, there’s sometimes a problem with login info that is saved to the keychain. If OmniFocus is launched as a result of a services request (basically any way other than clicking the app icon), it will not be granted access to its own saved login information. We have been in contact with Apple regarding the issue, and are currently awaiting a fix.
To avoid this problem, you’ll want to make sure that OmniFocus is already running when you try to use Clipping or a bookmarklet. If you do run into repeated prompts and have verified that you’re entering the correct information, quitting and restarting OmniFocus should solve the problem.
In some cases, it may be necessary to also delete the keychain entry that OmniFocus saves using the Keychain Access application (found in your /Applications/Utilities/ folder), and then create a new one by entering your credentials the next time OmniFocus syncs and making sure the “Remember in Keychain” option is checked.
Check that you are entering the correct account name and password, with the correct case. Passwords are always cAsE-SeNsiTiVe, and some servers (including the Omni Sync Server) treat account names as case-sensitive as well. In the case with Omni Sync Server, your entire account name should be all-lowercase.
If you have any trouble with these steps, please don’t hesitate to email or call (+1 800-315-6664) our Support Humans. We should be able to help you get back up and running quickly.