Problems when sending form

Hello and thanks for this nice plugin.
We are having problems using it.
WP 4.7.2, Hestia free theme. The form is integrated into the home page footer and it should use gmail smtp. Permissions are set to use less secure apps.
When we click on “submit” it waits some seconds, then it returns to the homepage (regardless of which “Thank You” URL we set, default page or “nothing”) with a http500 error.
Nothing is sent to the receiver.
Any idea? I don’t understand if the problem is related to gmail smtp or to “Thank You” URL.
Thanks in advance
AO