Error 20 The proxy failed to connect to the web server, due to TCP connection timeout

We have just run into a problem in our schools over the past two days.  We have a 100D to connect all wired devices to the internet and a 200F to connect all wireless devices to the internet.  On the 100D, we have no ability to connect to our SIS (School Information Services where we keep all school data such as attendance, grades, schedules, etc)  The error we get is : Error 20 The proxy failed to connect to the web server, due to TCP connection timeout.  This is only on our wired machines.  I can switch my laptop from wired where it does not connect to wireless where it connects with no issues.  I have tried pinging and trace route when wired with no issues.  What can I do to resolve this?