Reinstalled xpilot on my Mac. Now my Vatsim password gets rejected

Do MyVatsim and the xpilot client use different credentials?

No. Suggest double or triple check that you’re entering your correct CID and the password you selected in March and successfully logged in with two days ago, or failing that, resetting your password again. Lots of folks accidentally enter a typo and/or try to use an old password. Not saying you’re one of them, but it’s a common issue, which is why I mention it.