On my live and on my test forum I run into an MySQL error, while trying to enable the Q&A captcha plugin.

First:
SQL ERROR [ mysql4 ]

INDEX command denied to user 'xxxx'@'localhost' for table 'phpbb3_captcha_questions' [1142]

SQL

CREATE INDEX lang_iso ON phpbb3_captcha_questions(lang_iso)

The Table captcha_questions is created (empty), captcha_answers and qa_confirm are not.

Later on:

Table 'xyxyxyx.phpbb3_captcha_answers' doesn't exist [1146]

SQL

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.

edit
I was able to reproduce the error on two other forums (but using the same server)
edit2
Just did a test again and updated the information