Although this is now widely supported (including Ubuntu), still some (old) Unix platform do not support login with space in the middle.
Use of RFC2307bis permits it
This say, I don't know if it can generate any issue with [uidnumber] (BTW, [uid] is something else
) but I believe that testing should be quite easy.
Can you try to import only users with not space in their [uid] (which is hat GUI call "user name")?