> Loging in as marc the backspace key stopped to work and stty erase ^H
> has no effect.

You might need
stty echoe
aswell.

>  Additional the man command did not work (no manual
> found for xxx). After a su man worked but the backspace key fails as
> well.

One of the man page directories is not world readable - was it
/usr/share/man/cat1 ?
so you want a chmod 755 <whatever directory it is>

> Working in a shell without a backspace is the hell. The only chance
> is to login as root and then typing the appropriate stty command. It
> had worked in single user mode without problems.
> 
> How can I enable the backspace permanently?

pop
stty echoe erase ^H
into /etc/csh.cshrc and /etc/profile
(not sure about the profile bit)

Patrick