It seems that the RDF's generated by Tiki are not Standard conform.
Validator
FIXED UserPageohertel
Update:
After a close look into the output, I reallised that their was a little Warning Message in the RDF ! ( Generated by PHP ). This was the problem why nothing got displayed.
So after removing the causing Line from tiki-setup.php everything works fine with the above user:pass combination.
(Line 1418)
if($feature_obzip == 'y') {
// ob_start("ob_gzhandler");
}