This is the solution to your problem: Windows don't receive mouse clicks after docking the laptop into a docking station - #4 by marmarek
Reason for not accepting mouse clicks: Your qube calculated the needed VRAM and added 8 MB (I think) as buffer when starting. Your external monitor is big enough so that 8 MB are not enough VRAM to serve the external display.
The docs about the buffer:
By default, it is as much as needed for the current display and an additional full HD (FHD) display (1920×1080 8 bit/channel RGBA). This logic fails when the machine has primary display in FHD resolution and, after starting some qubes, a 4K display is connected.