Form not sending on Edge, Firefox

I've received several complaints on one of our forms [https://eqa.es/webinar-bonos-instrumentos] not working. After some investigation, I've noticed that when submitting from Edge and Firefox, the form isn't sent (reply is empty array), but it works fine on Chrome. We've been working with Chronoforms for several months and this is the first time this problem is brought to my attention.
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I checked for this error and found this page: http://www.chronoengine.com/faqs/70-cfv5/5259-my-form-doesn-t-submit-send-emails-save-records.html
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It refers to CFv5, though. So I cannot follow the instructions there. I've tried submitting from https://eqa.es/index.php?option=com_chronoforms6&chronoform=webinar-bonos-instrumentos , and only got a success 10% of the tests. Submitting from https://eqa.es/index.php?option=com_chronoforms6&chronoform=webinar-bonos-instrumentos&tmpl=component works 100%, so far. There are no other indications to follow.

what happens when the submit fails ?

The page with the form reloads. Debug mode shows answer is an empty array.

Tried to test the form on Edge now and go this:
​[h4]403: Access Forbidden
This form has not been submitted because your IP is listed in a spam database. If you feel this is an error, please contact the webmaster. [/h4]