Hi,
I moved the forum from a previous hosting to a cloud with virtualmin. Everything went well and the forum worked regularly.
Then I tried to upgrade from version 4.7.6 to version 4.7.18, but I received this error: 

We were unable to start a PHP session. You will need to contact your host to adjust your PHP configuration before you can continue. The error reported was: Return type of IPS\_DateTime::format($format, $memberOrLanguage = null) should either be compatible with DateTime::format(string $format): string, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

So I installed the forum (always version 4.7.6) on a test server, I searched the community and tried all the solutions proposed by other users, but I had no success (I changed options in php, I installed the update files manually, I removed all templates and all external applications. Nothing, it doesn't work)
As a last thing I uninstalled everything and tried to install a fresh version, on the test server, and this works regularly.
I would like some suggestions on what to do to bypass this problem and update the forum.
On the server there is php 8.1, I used the checker and everything is green

Would ensure that all files are up to date for 4.7.18 and that you're running PHP 8.1.29. 

You will also need to check your PHP session configuration on your server to ensure that it is valid.

 

Would ensure that all files are up to date for 4.7.18 and that you're running PHP 8.1.29. 

You will also need to check your PHP session configuration on your server to ensure that it is valid.

 

I downloaded the files directly from the client area and uploaded them. The php is 8.1.29

What should I check, in detail, on the PHP session configuration? I think everything is fine, because installing a fresh version does not give any problems, but if you have any suggestions I am happy to check

 

I downloaded the files directly from the client area and

You would need to ensure the files overwrote correctly.

 

What should I check, in detail, on the PHP session configuration?

That it is configured correctly for your server. 

I found the problem. In the previous hosting there was cPanel and it had created two ini files, in the upgrade directory, php.ini and user.ini, in which was set session.save_path = "/var/cpanel/php/sessions/ea-php72" , a directory that does not exist in the current server. I deleted the two files and now everything works fine

Glad to hear everything is working correctly for you