Error establishing a database connection with PHP 7.2

php5.6-mysql is already the newest version (5.6.40-38+ubuntu20.04.1+deb.sury.org+1).
php7.2-mysql is already the newest version (7.2.34-8+ubuntu20.04.1+deb.sury.org+1).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

The Apache Error log shows nothing.
The Access log shows: “GET /wp-admin/ HTTP/1.1” 500 3563 “http://www.xx.com/wp-admin/” "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) etc…

The Error I see on the website is like the headline says.

What do you want me to check?