I didn't got those trouble, but i'm still at Version 3.1 ;-)
But by the way: i hate this lousy readable CAPTCHA stuff and didn't understand why the script aren't simply check the time between load and send - i.m.h.o. it is enough protection-potential in there to figured out what messages are from Bots (because the full-fill all fields in milliseconds)?!
But until your captcha didn't work, why you are not working with the quiz-feature and define your own questions like this:
<h2>Bot protection</h2>
<p class="secure">please calculate or answer those simple questions - just to go sure that a human sender wrotes:</p><p class="quiz">[quiz random-capital-quiz 3/ id:qatest
"2+3 are |5"
"3 times a 3 results |9"
"17 minus 8 should be ... |12"
"how often is 1 in 3 |3"
"4 x 25 is |100"
"how many 5 found in 20 |4"
"6 Sisters playing with 3 neighbors. How many children play |9"
"half of 100 Meters are |50"
"how many drivers a bike can have |1"
"radical of 9 is |3"
]<div class="knopf">[submit "Send"]</div>
… and place it before (or better instead of) sour Send-Button and after your last input field.
Just as an nice hint how to handle it in a fast and small manner