All,
thank you for good ideas and input.
I managed to log in with username "admin" and my domain administrator password (note: i was not using the domain username, only the password). Weird, but it actually worked, might be a bug? (The old administrator password for the symantec authentication account is different to the domain administrator account, so I have not mixed these two together.) After having successfully logged in, i changed the login type back to symantec authentication, logged out, and logged in again with admin-admin, which worked fine. (I did run resetpass at an earlier stage in one of the attempts to fix the problem).
Case closed.
Best
/Nils