XOOPS 2.4.5 not allow to regist name with space.
Solution by Ghia:
replace line:
$restriction = '/[00-040]/';
with
$restriction = '/[00-037]/';
in class/userutility.php file
http://www.xoops.org/modules/newbb/viewtopic.php?viewmode=flat&type=&topic_id=65919&forum=7
Moved to feature requests
This is not a bug but a feature request.
For smooth operation the check should be extended so that at least the minimum alphabetic characters are present, else you could end up with user names of 6 blanks.