When filling out a validated form, the form successfully submits. However if a field error is thrown and then corrected, the form seems to hang with the spinning ajax gif by the Submit button.
Since JQuery 1.2, the "@" sign was depreciated as part of the Selector Attribute Filter syntax(see API). Which is why ALL hidden form values were replaced with the Captcha value instead of the specific one indicated by the attribute filter.
Simply replace mm-forms.js, line 70
jQuery(data.into).find('input:hidden[@name="_mmf_captcha_challenge_' + i + '"]').attr('value', match);
jQuery(data.into).find(':hidden[name="_mmf_captcha_challenge_' + i + '"]').attr('value', match);`