Hi constantinos.rokkos,
First of all, take my apologies for the delay in answering because of the weekend.
I enabled debug log and there is an error:
PHP Fatal error: Uncaught Error: Call to undefined function PHPMailer
This error is referring to the default PHP Mailer that most systems will come preinstalled with but it looks like your system does not have the default mailer and the only way to fix this would be to install the PHP Mailer library but this will require access at a server level and you may need to contact your hosting provider to help with this.
You also should deactivate all plugins (except Required Plugins) and then reactivate them one by one. This makes it very easy to isolate the plugin causing problems.
Please let me know how it goes.
Regards,
.
