A "best guess" would be that they are using something other than lower case letters and numbers as their username, something that was changed with version 4.1.11, and there is no specific message telling them that this is what they are doing incorrectly.
See the threads at
http://forum.x-cart.com/showthread.php?t=42147&page=4
and
http://forum.x-cart.com/showthread.php?t=41583&page=4
for more information on this and several remedies.
To answer your question directly, there are no mods that I am aware of that give you more information as to why a customer is having problems logging in or registering. You can add a "please email us if you have problems" type link so that they can vent their frustration that way, but I think what I mentioned previously will go a long way to eliminating most of the problem.