The problem comes from the fact that it is NOT necessary to add the form_id and Captcha method in the Captcha Configuration when you use a custom form.
If you do this, when the form is generated on the page, the Captcha module is called TWO TIMES instead of only one, and this causes some troubles in the database, because when you call the form page, and when you submit it, TWO lines are created in the captcha_sessions table instead of only ONE !!
So, you will have :
Line number | Token | Code
------------------------------------------------
N | AAAAA... | TT7Zi <=== FIRST CALL
N+1 | BBBBB... | dccedb00fa1234567890... <=== SECOND CALL (ABNORMAL)
Then, when you submit, a new line is created in the captcha_sessions :
Line number | Token | Code
------------------------------------------------
N | XxXxX | Zjj7R ( UPDATE of the whole former TT7Zi line )
N+1 | BBBBB... | dccedb00fa1234567890...
N+2 | CCCCC... | 542dda8e8d7fda2358b4b2f319ef198b <== SECOND CALL !!
That's why the generated WATCHDOG error line says :
user answered "TT7Zi", but the solution was "542dda8e8d7fda2358b4b2f319ef198b".