Press the DPI switch button once and move the mouse. If the cursor speed changes, you’ve found the cause. Cycle through the ...

It is possible that there is an accumulation of dust on your scroll wheel due to which the middle click is not working. Another cause is the corrupted mouse driver.

You imply that you didn't switch keyboards or mice. Since the problem occurred, have you tested scrolling on your built-in mouse without the KVM switch? Maybe it's the EDID programming that has ...