Question Probably an incompatibility between webtrees PHP and server PHP - what to do?

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You have directories 'app' and 'includes' which indicate it's the 1.7 branch of webtrees. That only runs on PHP 5.3 - 7.3.
It is possible your web hosting provider has switched PHP to another version (v8.x??) 

1) You are using webtrees 1.x

You can find the exact version by looking in the file /includes/session.php for a line like this:

define('WT_VERSION', '1.7.20');

2) Your webserver is running PHP 8.1 or higher.

My guess is that your host just upgraded.

Now is probably a good time to upgrade

See webtrees.net/upgrade/

Make a backup first, if you can.

Thank you very much for your answers.
Yes, I have the 'app' and 'includes' directories.
Actually my web hosting provider has changed PHP to version 8.1.
It has now allowed me to change to 7.3 and the web site is up and running.
In /includes/session.php I have version 1.7.9.
Yes, I guess I will have to do an upgrade.
I need to remind myself how to back up.

On webtrees.net/upgrade/ it says that the upgrade has to be done manually.
On my site it says
"A new version of webtrees is available. Upgrade to webtrees 2.1.20 ."
I can click on the link Upgrade to webtrees 2.1.20 (myaccount.domain/admin_site_upgrade.php).
That is, there is an automatic upgrade.
Am I mistaken?

yes, you are wrong. there is no auto way. The corresponding doc at  upgrade  shows you the way.

> "A new version of webtrees is available. Upgrade to webtrees 2.1.20 ."
> I can click on the link Upgrade to webtrees 2.1.20 (myaccount.domain/admin_site_upgrade.php).
> That is, there is an automatic upgrade.
> Am I mistaken?

The upgrade server looks at your PHP version. If it saw PHP8.1, it would assume you are already running webtrees 2.x, and hence it told you that you could upgrade to 2.1.20

But you can't. There are some manual steps required. See the referenced documentation.