I have a Windows 2008 web server on running IIS 7 with about 25 websites. One of those sites is an SaaS application that is accessed constantly throughout the day. However, one particular client keeps getting blocked from my server. They will be using the service, and then all of a sudden they cannot access the program, or any other site on the server. The entire office of 4 users is blocked from accessing anything on the web server. A trace route reveals they get all the way to the server before they are blocked. However, they can access a linux server that is a different VM with a different IP on the same physical server. Also, when they are blocked from their office, they can still access the site from their mobile phone or local Starbucks. They can also occasionally reset the router and gain access to the web server again as they are on a dynamic IP address.

I checked IIS and allow all IPs to access the server. There is nothing in the logs the says anything about a user being banned. I really have no idea what is causing this? Could it be a virus on their end? I have even moved the SaaS to a completely new server in a different location, and they were working fine for about a month, and then the problem started occurring again.

Are there any hidden blacklists in IIS? Or is it a routing issue on their end?