I brought this up in my other thread but felt it best to make a new thread to address this rather than piggyback on my other issues.

It appears that staff cannot login if their username is all numeric. (standard users do not seem to have this problem)
I deleted and recreated accounts several times to test this.

john (valid)
john42 (valid)
x123456 (valid)
123456x (valid)
123456 (unable to login)