Quite possibly it's a bug in phpBB's authentication model/bits, and may end up using cookies temporarily to store the answer to the captcha (for validation) but the password you enter is still incorrect. I don't see anything in phpBB 3.0.11's changelog
(we run 3.0.10 right now) that indicates anything was fixed relating to any of those things.
I can assure you 100% the captcha questions/answers/model works just fine.
Since the time your account was created, you've posted from 5 different IP addresses. The last time you changed your password was in August 2012 (although this was also when the board was migrated to a new server, so that may not be entirely accurate).
The best I can do is to force an account reactivation, which requires you to check your Email and visit a link provided there + set a new password. Alternately I can set your password to something temporarily so you can fix all this yourself, or update your Email address for you + force a reactivation. Let me know, either here or via a PM.