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Hello Falke,
Please turn on your debug php function and report the error message here.
I have no problem with Myguestbook, myself.
As for the CSS problem, you need to update the correct components within the stylesheet in order to obtain the correct colors (but you probably already knew that). If you go here:
http://eirenicon.org/modules/mydownloads/viewcat.php?cid=9 and download the silver theme, you will get a heavily commented theme and stylesheet-- perhaps this will help.