Sorry but I don't understand: how can be possible that I have problem with the wp_mail function but if I create a new account in a forum, I receive a registration confirmation email? Does this use mail function as well?

Thank you for your explanation 

Riccardo

Number 3

And please ask your hosting support to update MySQL to 5.6 version. The 5.55 is too old.

1. Ok I see the error and you see too. Please follow to that error instruction and contact to your hosting service and let them know that the PHP mail() function doesn't work.

If you've configured some SMTP plugin please follow to this support topic: https://gvectors.com/forum/how-to-and-troubleshooting/plugin-incompatibility-with-sending-notification-mail-through-smtp-server/#post-12929

Here are the 3 screenshots of your suggestion. It seems a problem related to the wp_mail function, but why then it worked with the email registration confirmation of the new account?

2. On the same admin page navigate to Tables sub-tab and find the _wpforo_subscribes table section. Take a screenshot and attach to your reply.

https://wpforo.com/wp-content/uploads/2019/01/wpForo-Subscriptions-Table.png

3. On the same admin page navigate to Server sub-tab and add it in your reply.

Well the translation issue is almost solved I can say, but unfortunately I get another kind of problem now! I have tried the notification system and it seems not working as it should do. I saw for example that I have been mentioned by @Robert and by you, @Sofy, and I received an e-mail where I could find the link to your post reply. Wow, I said, is exactly what I need for my website! But, my happiness has lighten down quickly: I have created another account with a different email address, of course, and I tried to mention my other account with "@" as you did with me here. But in the email address nothing came up, no email notification of the mention, with my great displeasure. So, I tried to do something similar using Contact Form 7 plugin, that it sends email as well, and I have received the email correctly. I have also noticed that even I click on the "Subscribe for new replies" button, a red popup comes out with the sentence: "Can't send e-mail confirmation" (and by the way, I can not translate this sentence with the phrases system 😫 ). So I simply disabled the e-mail confirmation for subscription, but the problem for the mention notification via e-mail still exists. Moreover, when I created a new account, I used the e-mail confirmation and it worked perfectly, so what could be the problem for the mentioning notification via e-mail and, less important, the e-mail confirmation for subscription?
Thank you again for all of your support and for your time, it's very important for all of us.

Riccardo