Hi Friend,
This is a basic use-case and it should work just fine. Please refresh web client once you add your domain user under admin group. Please also check your domain is up & running with no any issue. However, I have never tried this scenario with upgrade from 5.1 to 5.5. Was it upgrade or fresh install (just need more clarification)? Please also post which build numbers 5.1 and 5.5 did you use?