XF 1.3 XenForo fault length of the user name

Hi , in first sry for my Bad english

My problem:
The registration is not there any more possibly ,
They marked use mind. 3 letters , but we use it ,and test it in ACP ,
to put on a new user, however, also this does not go .
Can help pleas ?
yep thats is it , and is not possible to add new user in acp.
The error : he marked :
XenForo fault length of the user name minimum 3 letters .
Hi everybody,
i'm a friend of BoomBoomUlli and i'll try to explain it in another way.

so the problem is that if someone wants to register in the forum there always occurs an error with following text:
(I try to translate it as good as possible)

"Please enter a username with at least 3 letters"
"please enter a valid username"

we tried a few times to register ourselfs with different names but it's always the same.

thanks for helping