Error Authenticating Proxy Account when Executing SSIS job
Just in case others have gotten here for the same reason.  Make sure that the account your proxy/credential is using is a user in its default database in SQL Security.

We went through all the other suggestions (checking that the password hasn't changed, switching to a local service acct for SQL Agent User, rebuilding the credentials and proxy, etc.) and until we made sure the account was a user in its default DB, for some reason, that was causing it to pass a Null SID when it was trying to authenticate to AD.