Now i am happy that i can use slip aswell (again) ;).
My problem has been a hardware problem. Slip seems to
have problems with some modems and/or some i/o cards. After
i found an internal modem with a 16550A UART slip works like
a charm. But let's go to other common problems i heard about:

- The connection is only one-way. Other machines than the server
  can't reach the client ?

This is an arp problem.
You have to add '/sbin/arp -s $6 xx:xx:xx:xx pub' to slip.login
(with newer kernels) or issue a similar call on another machine
somewhere on the net. This only works locally (of course) if your
linux slip server has an ethernet card with the needed hardware
address.

- A client can only ping/telnet/... to the server, but not
  to other machines on the net ?

This is a gated problem. Alternatively you can enable IP forwarding
in newer linux kernels.

- Sliplogin doesn't start ?

This is mostly a permission problem. You should have a group slip
and put all slip users into that group. Also in doubt use 'make install'
instead of copying the files by hand. Then slip.login and slip.logout
have to be executable for the group.
You often find very useful hints in the logfile which could reside
in /usr/adm/messages.

- Sliplogin still doesn't start ?

Then i can only say RTFM !

- Something is wrong, but you don't know what ?

a) Check the client settings. Sliplogin uses by default a MTU of 1500, so
   must the client.
b) Are you sure, you enabled slip/cslip in the kernel ?
c) Are your line settings ok ? Device name, flow control, speed ?
d) Are your net-tools up to date ?
e) Did you pay your phone bill ?