Sounds like some clients/computers have problems with the certificate. I have tested from about 5 different locations and computers and 3 different clients and its working perfectly everywhere both with http and https so I think the problem is on your side.
Make sure your computer / servers date is correct as that may affect the possibilities for it to accept a security certificate properly.
Verify System Date/Time Settings.
1 Select Start > Settings > Control Panel
2 Double click Date and Time
3 In the Date/Time Properties window, set the correct date and time, and then click OK.
If you are on Windows XP it is also possible that you have to update the root certificate for your computer.
Update your computer's root certificate list (a solution for Windows XP users only)
If you use Windows XP, and you've tried the solutions above and you still can't connect, you may have an outdated SSL certificate. Windows XP contains an optional system component called Update Root Certificates, which, when necessary, connects to the Microsoft Update web server and downloads certificate updates. If this component is not installed, you need to periodically update the certificate list yourself. Here's how:
Click this link
http://www.download.windowsupdate.com/m ... otsupd.exe.
When prompted, click Run to begin the root certificate update.
Note: Please contact Microsoft customer support if you have any questions about the update program. You can also learn more by reading this Microsoft FAQ:
http://support.microsoft.com/kb/931125. Look for the section titled, Root Update Package (intended for Windows XP only).
If you use IE9 maybe this can work:
1. Open IE9
2. Go to Tools & Internet Options
3. Click on the advanced tab and scroll down
4. Under Security, uncheck the box 'Check for publishers certificate revocation'.
5. Uncheck the box "Check for signatures on downloaded programs"
6. Uncheck the box "Check for server certification revocation".
7. Click on Apply and OK.
