.htaccess Stopping Me Logging Into A .php Script

I hope I am on the right forum.

I have a site which is 99% on a secure sub-domain, and I was advised to put the following .htaccess in the sub-domain root to force all calls to https://

RewriteEngine On
RewriteCond %{SERVER_PORT} 80
RewriteRule ^ https://secure.mysite.com%{REQUEST_URI} [NS,R,L]

Everything appears to work properly on the site except for when I have to log into the admin area of a (commercial) php script. I just keeps resending the login screen to me. If I delete or rename the .htaccess, I can log in normally.
The developer has no idea how to fix it, and I wondered if this was a common problem with a quick answer.

Thanks,

Please see: .htaccess SSL Examples for your answer.