Do you have a proxy on your network ?
yes we run a proxy on our network.
When we have seen this issue before it has been due to an iis or proxy change. This article may be of interest to you to try on one of the affected users machines:
You might also consider recycling the framework application in config centre (7.5) or running an iisreset if you haven't tried this yet.
I had the same problem, my compagny use a PAC configuration file, but to make it work, i have to change in IE options/connection/network settings/ the proxy address, port and i check the boxe don't use for local addresses.