I think i figured out the error.
In Member Controls, Set Reserved Names ...
I was setting different words that people couldn't have in their nicks (defaults Admin, Webmaster). I think i accidentally entered a space. When i went to the last one .. and deleted the stuff afterwards (by hammering the delete key) all was well.
And people are now able to register. ! I think the interface to enter stuff here is a little problematic.... or the program should do a check to verify your list of unwanted words somehow.
by the way .. this is line 185 in register.php
if (strstr($namecheck,$reservecheck)) { fatal_error("$txt[244] $reserved"); }