Insufficient privileges (ORA-01013) "connect / as sysdba"

As the header says, I get a
ora-01013 (Insufficient Privileges) when I do sqlplus /nolog
connect / as sysdba

Platform:	HP-UX (itanium), 64bit, V11.23
Oracle:		10.1.0.4.0

Everything here cries "wrong group, wrong group", so that's what I went to the sysadmin with.

There's a little history here:
it used to work last week, on server A, in a package. Last weekend, the package was switched to server B. I already noticed the groupid was missing, that is, instead of seeing oalg01:algdba, I saw oalg01:647 (numbers may be different, but are not important).

So, obviously, something has gone wrong. The ownership was restored, according to sysadmin.
However, I did not get it to logon without the dreaded ORA-01013, so finally, I asked the sysadmin to drop and recreate the user.

Reinstalled, repatched, still ORA-01013.... When I "id" the user, I get the correct primary group (algdba), so things look ok.

Any thoughts, other checks I can perform before dropping group and user?!? What does oracle do behind the scenes to check the group?

-- 
Regards,
Frank van Bortel

Top-posting is one way to shut me up...