Blank page is common after a 500 server error.

So, what does your .htaccess now contain? Also, what is inherited from a parent directory's .htaccess, if so?

However, the "Parse error: syntax error, unexpected '?' in /<root>/<dir>/htdocs/piwigo/piwigo/include/functions.inc.php on line 628" indicates that your PHP can't parse the

    $performed_by = $user['id'] ?? 0;

with its ?? Null Coalescing Operator, which is odd because that was added to PHP 7.0 already, so are you sure you're not running a very old PHP version, no matter what you do in .htaccess to select a different version (what even do you do there to accomplish that)?

First, you do not want to use AddHandler that way, because the extension specified is matched against any extension of a file name, even for multiple extensions, e.g. it would match also file.php.png and you certainly don't want an arbitrary uploaded "image" file that contains PHP instead to be interpreted as a .php file. Unless you have an explicit handler for .png as well, in which case the rightmost handler is taken (which you would need to define for every possibly up-loadable extension).
See https://httpd.apache.org/docs/2.4/mod/m … addhandler and https://httpd.apache.org/docs/2.4/mod/m … ultipleext and https://www.php.net/manual/en/install.unix.apache2.php figure 8.

Also, AddType is unrelated to how the server processes files, it tells the client what MIME type an extension's content is if served as is. But the type will be used as handler-name if a handler-name is not set, so you might even confuse things by using different values for AddType and AddHandler, I'm not sure.

You may want to use something like

<FilesMatch "\.php$">
  SetHandler application/x-httpd-php
</FilesMatch>

instead, see also https://httpd.apache.org/docs/2.4/mod/c … sethandler

However, whether including a PHP version number as in x-httpd-php81 would work for you at all (I assume you read that somewhere on the internet but unrelated to your setup i.e. it sounds somewhat like cPanel) I don't know, that depends on how the web hoster set up the environment. Also make sure the correct PHP module is loaded, for example
LoadModule php_module modules/libphp8.1.so
in the Apache config (e.g. /etc/apache2/httpd.conf) is present.

You may get better specific advice at https://serverfault.com/