in the German language umlauts are used very often. If there is a non standard letter in the answer (e.g. "Käsebrot"), it will be decoded as question mark ("K?sebrot"). To fix it, you should replace
'AB_Question' => strtolower(request_var('AB_Question', '')),
with
'AB_Question' => strtolower(utf8_normalize_nfc(request_var('AB_Question', '', true))),
in ucp_register.php. This should be in line 124 in install.xml.
Regards
Matz