At the truth i didn´t solve this because i need to use numerical user name... I tought that developers would do some thing about this...
I'm not aware of them having any plans to change this behavior at thsi time,
Try starting with a letter then add the numbers, I wonder if it's a part of a security complexity. Not sure, but worth a try.
I am pretty sure that it's because the ticket system assumes a string of numbers like that is a ticket number.