Blank page after PHP security updates

[BIMAQ-Admin]

After the PHP security updates at wednesday (26-10-21) evening, our webserver (running on Ubuntu 24.04.5 LTS) was blocked by an "Internal Server Error"

Due to an issue with /etc/ssl/certs/ca-certificates.crt the ubuntu update package installation partly failed that day.

Rolling back to a snapshot made immediately before the ubuntu updates, we could update the ProcessWire core from 3.0.246 to 3.0.259 and solve the package installation issue, but since then the webpage is completely blank:
.htaccess was not changed: renaming .htaccess on document root and .site/ did not produce any messages
index.php on document root maybe changed during PW update - it is dated  Aug 28 13:47 -  and the version on .site/assets/ seems to be the original from Aug 16  2024.

In /var/log/apache2/error.log there are warnings like:
PHP Warning:  PHP Startup: Unable to load dynamic library 'dom.so' (tried: /usr/lib/php/20230831/dom.so (/usr/lib/php/20230831/dom.so: undefined symbol: zend_hash_real_finit_packed), /usr/lib/php/20230831/dom.so.so (/usr/lib/php/20230831/dom.so.so: cannot open shared object file: No such file or directory)) in Unknown on line 0
despite those files are present there:
root@bimadmin-www:/var/www/html# ls -l /usr/lib/php/20230831/dom.so
-rw-r--r-- 1 root root 207112 Oct 19 17:16 /usr/lib/php/20230831/dom.so

When I replace index.php on document root with the previous version  I see following message on red background:

Oh no… Fatal Error: Uncaught Error: Undefined constant PDO::MYSQL_ATTR_INIT_COMMAND in wire/core/WireDatabasePDO.php:286

#0 wire/core/WireDatabasePDO.php (286): constant()
#1 wire/core/ProcessWire.php (541): WireDatabasePDO::getInstance()
#2 wire/core/ProcessWire.php (316): ProcessWire->load()
#3 index.php (52): ProcessWire->__construct()
#4 {main}
thrown (line 286 of wire/core/WireDatabasePDO.php)

This error message was shown because: site is in debug mode. ($config->debug = true; => site/config.php). Error has been logged.

It looks like index.php is the cause - does somebody have an idea for a solution?

Both versions of index.php are uploaded.

Thanks & Regards

index-3.0.259.php index-3.0.246.php