I was able to solve the problem as follows: In /wp-content/plugins/classicpress-seo/includes/helpers/class-param.php are four methods in which $filter = FILTER_DEFAULT, $flag = ‘’ is defned. A 0 must be used, not an empty string.
After I made this change I could activate and deactivate the plugin and activate it again. All the previous tests worked. Do more tests. Hope that tomorrow I can use PHP 8 on the productive system.