It did not take me long to determine the cause of the problem - I gathered the following from several internet postings
A number of our members have reported receiving the "You exceeded the maximum number of login attempts" message while trying to login to the forum, and are then prompted to enter the CAPTCHA code as well as their username and password.
Unfortunately it seems that several phpbb based forums have been attacked in the same manner which involves a bot persistently trying to login to member's accounts. The forum software catches this and after 3 attempts prompts with the CAPTCHA code.
There is no indication that the bot has ever got past the CAPTCHA code as it would require both the correct password, and the correct CAPTCHA code.
However, if you have a "weak" password, we would recommend that you change it to something that would be much more difficult for a bot to guess, using either a dictionary or brute force attack.
Recommendations for increasing the strength of your password are using a combination of letters and numbers, using upper and lower case letters, and adding non-alphanumeric characters (i.e. *&$% etc.)
Sorry for the inconvenience.
If you check the box with "Log me on automatically each visit" while logging in and you don't log out when visiting this forum, then you also won't get these annoying messages.
Also, please consider increasing the strength of your password as stated above.