Possibly AOL
The answer may lie in the blurb on the page when you log in. A precis is below:
<<This is a special announcement from PPRuNe Forums Admin. . . . repeated hack attempts . . . (HackBot) . . . malicious and nuisance
. . . the HackBot is intended as a Denial of Service (DoS) attack.
As the computer running the HackBot is using an AOL IP address, after it has randomly tried to access an account, the account is locked out and the IP address blocked for 15 minutes. If another AOL user with the same IP address tries to log-in within the 15 minutes of lock-out, they will get the locked out message too.
In order to stifle the HackBot, we have had to block the IP address being used which has caused more inconvenience to some AOL users. For this we apologise but it is the only way we can prevent the HackBot from running. We have also introduced a 'Captcha' image code section to the log-in which is designed to make sure that it is a human rather than a Bot trying to access PPRuNe. The Captcha image code is turned on and off randomly and you may or may not encounter it.