After moving my characters Profile & Target to my preferred location directly above my Toolbar.
I get these Lua Error Messages when mounting another players “Multi Passenger Mount” or a “Quest Giving Mount”:
With No Addons Installed: “Your AddOns…Lua errors…”
With Addons Installed:
Lua Error – Interface\FrameXML\AnimstionSystem.lua:61:… PlayerFrame:SetPoint.
Found this, seems to be a known issue. No fix yet but you can disable lua errors entirely to avoid seeing the errors if you like. Just make this a macro to toggle the display off and on: