Hello brian,
I'm really really sure. This is very strange.
I have exactly this on my htaccess:
Order Deny,Allow
Deny from all
Allow from ::1 127.0.0.0/8
Allow from 123.456.789.10
I have tested this same rule on other folders, and it works normally. But in the admin folder of whmcs no, continue accessing the index.
However, it block static files, images, css.
And it does not allow login, because it block the dologin.php file.
Apache is configured correctly, with:
AllowOverride All
Note: I use a custom name for the admin folder.