I cannot change private message buttons

As the title suggests folks. I have been trying to change the buttons on my forum, they were going ok until i tried to change the private message buttons. when i try to save the new button codes I have entered i keep getting a pop up saying "you must provide a menu name" There is also a menu name entered and I am unable to alter it.......

Any ideas what I do folks

Much appreciated

A-CP => Display => Pictures and Colors => Pics management >> Advanced Mode >> General / explore scroll down and change it there and save.