So, after many problems I decided to do the Erase and Install (I had done the Update before). Everything went fine after that.

Then I downloaded the newest software (iTunes, Quick Time, Login & Keychain). During the installation process you need to reboot. During (or after) the reboot I get a warning that my boot partition needs to be rewrtiiten. A little later I get the login screen. So far so good.

Now if I login, Finder won't start. I just get the eternal rainbow thingy. I can launch the programs in the dock. They will work or not, depending on the order in which I start them. Unix Terminal won't work if I open it as the first program, but if I open Firefox first and then Unix Terminal, both programs are working fine. In the top bar I have the airport symbol, the language symbol, the loudspeaker symbol but not the time or date.

Please help me somebody! I am running a G4 (non-Intel). Thank you very much.