Cant send pm to certain members

Hello,
I'm an administrator for my forum, recently some of my users complaint that they cant send pm to certain members that have a username like ---->> Orga. So after they try to send the pm, this error pop out "Sorry, but no such user exists.". Only some user faced this kind of problem.

Some of them also cant view the memberlist even at the admin panel, the memberlist are enable for all users to view.

This is the link for my forum :
http://www.wiratunggallegacy.com/

Thank you.