Comment 3 for bug 223945

You can try to temporarily bypass the KVM switch and plug the mouse straight into a USB port.
I don't remember the PS/2 specs well enough to know if it can even send extra button values from USB mice.
It can also be an issue with the adapter: it doesn't handle extra buttons correctly.
Another problem is that PS/2 doesn't register USB product and vendor IDs correctly, which is how btnx detects mice.

I'm marking this bug as invalid because it's not an error in btnx's behavior.