On my live and on my test forum I run into an MySQL error, while trying to enable the Q&A captcha plugin.
SQL ERROR [ mysql4 ]
INDEX command denied to user 'xxxx'@'localhost' for table 'phpbb3_captcha_questions' 
CREATE INDEX lang_iso ON phpbb3_captcha_questions(lang_iso)
The Table captcha_questions is created (empty), captcha_answers and qa_confirm are not.
Table 'xyxyxyx.phpbb3_captcha_answers' doesn't exist 
INSERT INTO phpbb3_captcha_answers (question_id, answer_text) VALUES (3, 'blibla')
I've fixed it with the stk tool.
Please find attached the whole bunch of different error messages I got.
I was able to reproduce the error on two other forums (but using the same server)
Just did a test again and updated the information