I just got everything to work.

The problem was that the SP4 setup deactivated all active protocols of the updated instance. I just reactivated "Named Pipes" and "TCP/IP", now everything works...

Yes, but it didnt change anything..

ok looks like the UPGRADE=1 parameter isnt a solution for this problem

After upgrading to SP4 using the parameter above, I cant connect to the MSSQL server

I get a "connection refused".....

Unfortunately the MSDE-Version of the setup doesnt have a GUI..

If i start setup with no arguments or from explorer, I get: The instance name specified is invalid.

I think this comes...

Thanks for the fast reply!

I try the installation from command line (what do you exactly mean with GUI?)

The server only has one (the default) instance running, and doesnt have SP4...