PHP version problems 7.4 move to 8

I have a domain and want to upgrade from PHP 7.4 to PHP 8.3 or any of the PHP versions 8. As soon as I do this I get locked out of my admin panel and cannot contact the site via any browser. If I upload a backup of he file and the database I can get back in.

I experimented a lot (and I am not that much of an expert in WP I haste to say) without success.

I expect there is somewhere an incompatibility. I checked with other folk who also run the same theme and it works fine under PHP 8. So I exclude this for now. I suspect a widget or maybe my ISP. My ISP confirms al is well on that end. So looking in to the widgets.

Here is what I did. I have a domain with only a test website attached to it. So, I activated the 2024 theme. I installed the most important Widgets I use on the other site. I ran a widget that checks PHP (PHP Compatibility Checker) and all widgets are OK for PHP 8. Including the ones that are not activated as only a few are activated.

FYI, the installation information in case it gives a clue.
Hostname für MariaDB 10.6 > mariadb106
Hostname für MariaDB 10.4 > localhost
Hostname für MySQL > mysql
Mysql version 5.7

When I test this setup by calling up the site in a browser it opens fine under PHP 7.4. Under PHP 8 it does not load, and my WP/Admin also refuses to reload until I switch back to PHP 7.4.

The PHP compatibility checker also creates a report whereby almost all the widgets there is an error in one or more lines. As I can not enclose files, here the link to see the report just in case it helps (expires in 30days), here: https://easyupload.io/ojgx3g

Now I also activity in wp-config.php > define( ‘WP_DEBUG’, true ); but do not get any reports that something is amiss.

I also reloaded WP latest version new in my control panel but it did not solve the problem either.

As I can not enclose files, here the link to see the report

Any ideas or suggestions?