Thanks Albert,
Jader Silva from SC support is on it. I gave him the php.ini and some applications.
And indeed I see a blank screen in Firefox and an HTTP 500 message in Chrome when I switch to 8.1. Everything is fine on 7.3 - generated under 9.9.007 (4).
I did a test on another vendor (he uses Plesk) and I have the same issue with PHP 8.1. So the problem is on the SC side and not on Strato as they said at first.
I joined SC because my developed software had been independent and could be installed at any hoster on world with
- Web space
- PHP runtime
- MySQL
It had worked perfect this way with SC in past.
The SC development team is testing wrong on there own server with access to the operating system.
On my delvelopment naturally all is fine under PHP 8.1.
If a VPS or Managed Dedicated Server is required for customers, so I could develop more elegantly with JAVA.