Hello,
check-conversionplugin.php uses is_readable (https://www.php.net/manual/en/function.is-readable.php) from PHP to check if it can access the main soffice exec.
Please check your server logs and follow the steps explained on https://www.phpdocx.com/documentation/conversion-plugin/debugging-libreoffice to debug it. We recommend you to test it using PHP CLI, to check if the problem comes from the path or the web server that is blocking PHP to read external or executable files.
Regards.