Shamelessly stealing my own words.
I'd advise starting with SystemPack (option #3), but since some people are opposed to installing additional software for various reasons, know that there are other options as well.
(The thread is for G1, but the fixes apply to both G1 and G2.)
1) check if you have any .RPT files inside/near the Gothic folder (e.g. if you have Gothic installed in E:\Games\Gothic, check if there are any .RPTs in either E:\Games or E:\Games\Gothic) and delete them, if you find any;
2) start Gothic twice (manually, not through Galaxy) and then kill rundll32.exe in the processes;
3) install SystemPack