Hi,
I normaly use Firefox 3.0.10 where I have this problem. After you wrote your answer, I also tried IE 7.0 and it works without the problems, so it happens only when using Firefox.
I heavily modified the source code, so I think that I damaged it somewere, but cant find where. I am still checking it. The other thing, that I must mention is, that after the error I deleted all the files from the server and uploaded the untouched freshly downloaded basic version and the error message appears (!).
Its a pity, because this is the best and the most beautiful web tempelate that I was able to find ever and using it will save me a lot of work with still a great results. I hope it will be solved. Thanks for your help.