Actually Google was right. There was a serious problem. Somebody managed to install a .php file on my blog.
It is called sys_engine9682.php or in a later version sys_engine4823.php. It contained an eval() statement that apparently pointed to a Czech website.
No fun at all, because I do not know, how they got into it and how I can prevent them to do it again. Could happen, that I have to do everything again.
No fun at all. Bastards. Should have used a Domino blog template.