When creating my 345th identity at our dental insurance company, I had problems creating a login ID (failed like the first 4 or 5 times until I looked at the rules:
Your user name must be at least 6 and no more than 31 characters. Both letters and numbers are acceptable, but no spaces or special characters may be used. Your username must not contain numbers only. If your user name is 8-character long, the first 6 must be letters and the last 2 must be numbers.I happened to had chosen a name with 8 characters the first few were characters and the rest were numbers and then changed that to all alphabetic (thinking it was a rule against numerics, then changed it to all lower case and finally hit the help button to see the above rules.
Note the special rules for when the user name is exactly 8 character (sic) long. Strange, very strange.