UPDATE: I hit level 26 and put a point into the primary ability I use and it changed my primary ability to my mount? I moved my primary ability from the power sheet back onto LMB and tried to mount and it worked. Perhaps skilling up your primary ability will fix the bug for this particular instance of it?
I'm having this same exact issue, I tested my other character with the old keybind and he can still mount so it must be a character specific bug. Going to use my laptop and see if it is related to a temp file stored on your computer or if it is saved on their server. Will post after I test. Referring to fart133 in terms of…