It seems that a a recent update provided by Microsoft to fix vulnerabilities in has broken fuctionality of the SSL VPN on Fortigate devices. If you see the following error messages when browsing to the login page you may be expierencing this issue:
Internet Explorer cannot display the webpage
The connection was reset
The connection to the server was reset while the page was loading.
No data received
Unable to load the web page because the server sent no data.
The update released by Microsoft is to fix a vulnerability in SSL 3.0 and TLS 1.0. The vulnerability could allow information disclosure if an attacker intercepts encrypted web traffic served from an affected system. More from Microsoft here
Fortinet’s customer support bulletin CSB-120117-1 refers to the issue. Custom firmware which includes a patch is available on demand from Fortinet support. . Forthcoming general releases will include this fix.