It is indeed pun_stop_bots, which is a shame because it's the only effective method to stop bots, since captcha recognition is passed like it was child's play nowadays and my forum gets tons of spam if I disable the smart questions of that extension but enable captcha.
So well, the problem lies in the fact that a hidden input is used for the message field, which is silly since a text message may as well include double quotes (", " html entity), which break the value="" definition to one character before the first double quote occurence (hence the message is truncated).
Easy but ugly workaround (it works, so who cares):
Where it reads:
<input name="<?php echo $hidden_key; ?>" value="<?php echo $hidden_value; ?>" type="hidden">
<!--<input name="<?php echo $hidden_key; ?>" value="<?php echo $hidden_value; ?>" type="hidden">-->
<textarea name="<?php echo $hidden_key; ?>" style="display:none"><?php echo $hidden_value; ?></textarea>
then refresh hooks from administration/extensions, and you're good to go.
HOW TO TEST:
Before workaround, try to post as guest a message containing one or more double quotes, it will get truncated before the first double quote.
After workaround and refresh hooks of extension, insert the same message, no more truncation.