You receive multiple event 23002 alerts within the event log. See below:

Node:          Servername
Message group: Alert24
Application:   Event Log
Object:        Application
Severity:      Critical
Text:          EventType: Warning, EventSource: Microsoft ISA Server Web Proxy, EventID: 23002,
EventDescription: ISA Server was unable to decompress a response body from www.domainname.co.uk because the response was
compressed by a method which is not supported by ISA Server. This happens when a Web server is configured to supply responses
 compressed by a method that is not supported by ISA Server regardless of the type of compression requested.
If you want ISA Server to block such responses, configure the policy rule’s HTTP policy to block the Content-Encoding header in responses.
 Otherwise, such responses will be forwarded without decompression to the client and can be cached.
You can cancel or reduce the frequency of the alert generated by this event in ISA Server Management.

Solution

A solution is to disable the compression filter by clicking configuration within ISA, add-ins and then disabling the filter.