Symptoms
After LSH -> NG Migration several websites show 500 Internal error. There are no user-uploaded content for these websites in their respective directories in the webspace. All sites have Wordpress content management siteapp enabled.
error_log of the webspace is full of entries like that:
/var/www/vhosts/13/133329/webspace/siteapps/WordPress-4633/htdocs/wp-content/themes/test/.htaccess: Invalid command 'php_value', perhaps misspelled or defined by a module not included in the server configuration, referer: http://domain.tld/
Cause
php_value directives are used in the .htaccess files. This directive is not supported by Fast-CGI/PHP-fpm which are used in NG hosting. Old mod_php supported that. It was dropped due to its incompatibility with MPM worker.
Resolution
Find the entries "php_value" in the site source code (most probably you will find them in wordpress .htaccess files) and comment the lines found:
find ./webspace/siteapps/WordPress* -iname ".htaccess" -exec grep -l "^php_value" {} \;
find ./webspace/siteapps/WordPress* -iname ".htaccess" -exec sed -i 's/^php_value/#php_value/g' {} \;