I use Keyclone and have a kinda similar problem.

I go to:Setup> mouse-map> and I input XBUTTON2 on my key "U" where this key switch my action bar1 with action bar2.
It doesn't work. But the most weird is, when i bind XBUTTON1 with the key "U", it works. I have unbound the XBUTTON 1 and 2 from wow key bindings and still the same prob.
The XBUTTON2 won't bind to the U. Any solution plx?

Thanks in advance,
Theo