Exchange 2010: every Friday HTTPS/Outlook anywhere breaks until i do an IISRESET

Like clockwork every Friday I come to work and get a couple of calls from external users (this is the only way i find out, unless i check my connection) that tell me they cant connect. I check my outlook connection and it has fallen back to TCP/IP. Now we have set our network to use HTTPS all the time by ticking both of those boxes in outlook (fast and slow networks). And this works fine until it breaks on thurs night/fri morning and falls back to TCP/IP, thus external users cant connect until i do an IISRESET.

We only have 30 odd users and all-in-1 exchange server, with half of these users working from home or traveling, so load shoudnt be an issue.

Any idea why HTTPS breaks once a week?