Web Portal Error after upgrade to v9.1. Can't login with identity's language being set (UID_DialogCulture not empty)

Hi all,

After re-installing version 9.1 web app (portal/IT shop), I noticed strange behavior when logging into a web portal. When logging in as identity (authentication is set to Employee role based) who has UID_DialogLanguage set to any language (tested with most English variations, German, Danish, Afrikaans, some other languages) - web portal throws the error "Object reference is not set to an instance of an object". When logging with the user who has no UID_DialogLanguage set - I can log in successfully. Logs show that user is authenticated successfully in both cases, but in case with the language being set it will error out. There was nothing else obvious in the web app logs I could spot. I have tested these users pre-upgrade and login worked without any issues. I have also installed extra language packs, but no success either. Anything obvious I am missing? Thanks in advance.