Bug in Language File OR Function

[wizzy]

Is there a debug.xml file in /data/other/ ?

no, but after activating the debug plugin I received a good hint:
"Headers were sent out already" error message
-> in the german language file there are 2 linebreaks behind ?>
-> deleting those linebreaks solved the issue
-> I attached the updated german language file in my original language file post

if you replace your lang file with en_us.php does the login then work?

yes - would solve, but no need to try this anymore ;-)