The error has nothing to do with this plugin, this error is telling you that there is a problem with your mail server or mail sending method.
1.- Go to this page:
http://www.mydomain.com/admin/settings
and double check that your "Site Email" is a valid email address, I recommend to use an "@mydomain.com" email.
2.- Go to this page and enable smpt
http://www.mydomain.com/admin/settings/smtp
After step 1 do a test: try to send a verification email with the top bar.
If the same error shows up, do step 2 and repeat the test.
If nothing of that works, please let me know what is the last thing you did before the error started to happen.
Did you updated the PHPMailer library or something else?
Are you sure that the email function is enabled for your server?
Senior Developer
https://wiki.oxwall.com/faq:troubleshooting-could-not-instantiate-mail-function
I hope it helps.