Google up Kerberos Double Hop. This is screaming this as the cause, but until you've ran into this headache and a half you're not aware it's hiding out there. My guess is the old login/group was setup via SPN and the new one(s) need this as well.
Never stop learning, even if it hurts. Ego bruises are practically mandatory as you learn unless you've never risked enough to make a mistake.
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