This might be relevant:
I found a solution sort of. The other day I cleared my browser of all history and cookies on my pc and when I went to log back into the forum, it asked me to solve a captcha, which I did. Then I tried my tapatalk from my iphone and I was able to login without answering a captcha.
Not sure if it's something you did or if it was from solving the captcha on my pc. The captcha was never visible from my iPhone.
For the same effect without clearing history you may try:
1. in browser, log out of forum
2. close all browsers and open new one
3. log in with username affected and junk password (just pound forehead on keyboard for maximum effect)
4. repeat until forum asks you to also solve captcha
5. enter correct password and solve captcha
6. log in with tapatalk, hope it doesn't ask for captcha