there are no end stops on the mk3... it uses motor current to detect an obstruction to identify the extremes of motion, perhaps you have too much drag on the assembly
with the power switched off, try lubricating the rails and moving the X carriage from side to side to distribute the lubricant and retest, if this doesn't work,
try removing the X motor and see if it runs without the belt fitted, if this doesn't work,
try swapping the X and Y motor connections on your einsy, if the same motor is faulty on both controllers, it could be a motor fault
if the fault moves to the other motor, it could be an einsy controller fault,
if the fault disappears, it could be a poorly seated connector... regards Joan
I try to make suggestions that I believe to be well founded and safe,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions,
what you do, is YOUR responsibility.