Maybe this helps or gives a hint to anyone:
I had the same problem when accessing my wordpress installation over a SSL proxy to improve security. My webhost does not provide self-signed SSL certificates for free so I cannot access my domain securely with https://<mydomain>. Instead I have to use https://<webhost-ssl-proxy>/<mydomain>. Now when I use this url https://<webhost-ssl-proxy>/<mydomain>/wp-login.php the wordpress login page comes up over SSL. When I try to add a hyperlink the popup window is empty. When I do the login without the SSL proxy adding a hyperlink works fine, the popup windows shows content as expected.
