The symptom was that I could login if I used my password manager browser plugin, but not if I pasted it from my password manager.
The problem was that this field was used to enter a 10-digit code, and as it turns out, on default Windows10 system, the fonts are set up so that this field only fit 8 of them. Oops! :)
Around the time of AOL3 or early AOL4 someone found a user name exploit.
When making a new account, on the client side use winapi's EM_LIMITTEXT to bypass the max character limit on the input textbox. Enter one or two letters, a bunch of spaces, then some more letters.
The server side would truncate to the original length, leaving you with a one or two letter username, working around the 3+ requirement.
Nitpick: if the number is higher than one, then it's at least two times too many.