Hi,
I've spent the last hour or so tracking this down, just thought i'd post this. I recently upgraded from version 7.01 -> 7.9. I found out there was an issue with the formatSQLInput function.
It changed during one of the versions and added the [ and ] characters which are now filtered. This causes a problem where people who have a username with a [ or ] in it. When they try and logon their username changes to the hex equivalent and when the database compare occurs it always fails the login due to them not matching.