The error was: utf8 "\xF6" does not map to Unicode

Validating kogentis.de <http://kogentis.de/>

While validating my site this error stops the process:

"Sorry, I am unable to validate this document because on line *65* it 
contained one or more bytes that I cannot interpret as |utf-8| (in other 
words, the bytes found are not valid values in the specified Character 
Encoding). Please check both the content of the file and the character 
encoding indication.

The error was: utf8 "\xF6" does not map to Unicode"

The source code doesn't contain this characters... it seems, it's a part 
code of thickbox-library...

Thank you for any reply

Michael