AW: Possible issue with UTF8 misconversion (username with umlauts)

I'd also check the Perl module you're using. Any non-ASCII data sent to a web URL is usually URL encoded... with % and hex characters. So it might not be anything in FreeRADIUS.

Good advice. I wasnt aware, that the Perl module isn't part of freeRadius. Found the issue there. Sorry for disturbing you.