javascript errors on entering/leaving form fields

Every time I focus/leave a field in the form, a javascript error is displayed in the browser console. The error is:

:formatted:191 Uncaught ReferenceError: mwd_delete_value is not defined
at HTMLInputElement.<anonymous> (:formatted:191)
at HTMLInputElement.dispatch (jquery.js?ver=1.12.4:3)
at HTMLInputElement.r.handle (jquery.js?ver=1.12.4:3)
(anonymous) @ :formatted:191
dispatch @ jquery.js?ver=1.12.4:3
r.handle @ jquery.js?ver=1.12.4:3

Also, if I fill the form (all fields with valid values) and then click on the submit button, this error is shown:

:formatted:191 Uncaught ReferenceError: is_email_valid is not defined
at mwd_check_required1 (:formatted:191)
at HTMLButtonElement.onclick ((index):113)

Everything else javascript-related seems ok. No load errors. At least no javascript-related load errors. The only error shown is about a missing image:

VM942 jquery.js:3 GET http://canzioneventos.com/wordpress/wp-content/plugins/wd-mailchimp/css/frontend/images/ui-icons_222222_256x240.png 404 (Not Found).

Thank you in advance.

Regards.

Víctor Márquez

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