The problem is not being caused by the BPS Pro RSK 10.5 filters.
The problem is not being caused by another plugin or your theme.
If I try to change anything at all about the standard generic WP code in your existing root htaccess file it is being blocked or reverts back to the generic WP htaccess code.
Example: Even manually adding BEGIN WordPress “test” is not allowed/is being blocked. It does not appear to have anything to do with any other plugins you have installed or your theme. This is a problem with either the server itself or a cPanel setting of some kind or maybe mod_security. So you need to contact your host support and ask them why when you try to change your root htaccess code the file/code reverts back to the generic WP code below. In other words, something is preventing you from being able to edit your root htaccess file and it does not appear to be coming from anything installed on your site and instead appears to be a server or cPanel setting or mod_security setting that is breaking things.
# BEGIN WordPress <IfModule mod_rewrite.c> RewriteEngine On RewriteBase / RewriteRule ^index\.php$ - [L] RewriteCond %{REQUEST_FILENAME} !-f RewriteCond %{REQUEST_FILENAME} !-d RewriteRule . /index.php [L] </IfModule> # END WordPress