This is usually when you have ModSecurity or a "Web Firewall" enabled on your hosting account.  Usually, your host can turn it off for the cmsb folder or the entire site if you ask them.

What's likely happening is the web firewall is too sensitive and returning a false positive for that URL and blocking it.

You can test this by trying shorter and shorter URLs until you find the shortest one that is blocked.  My guess is it's the ?filename=.htaccess part or just ?.htaccess

/admin.php?filename=.htaccess&menu=_codeGenerator&_generator=permalink_codeGenerator

I get:

Forbidden

You don't have permission to access this resource.