Telnet Login - admin credentials fail

Dear all,
I managed to get my Kobo basically running again.
However, with telnet activated in NickelMenu, I can yet connect to my kobo, but the credentials "admin"/admin and "kobo"/kobo both fail. (Login incorrect)
Previously, admin/admin has always been working fine.
Any hints, what I am doing wrong? How could I change the credentials?

Is there also a way to connect via SERIAL Com port? Might be more stable...

Thanks for any hints!