TMG Server: Getting "13 The data is invalid" for HTTP requests from device

I'm getting "13 The data is invalid" for HTTP requests from a device our school has been given by our electricity company to monitor electricity use. I was told it only needed port 80 out to the Internet, so set it up as a firewall client (gateway as TMG server).

Here is what appears in TMG log:

Failed Connection Attempt SRV-FW 26/09/2013 13:20:16
Log type: Web Proxy (Forward)
Status: 13 The data is invalid.  
Source: Internal (
Filter information: Req ID: 09ec2e5a  
User: anonymous
 Additional information
Object source: (No source information is available.)
Cache info: 0x0
Processing time: 0 MIME type:  

Thanks in advance!
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Zaheer IqbalTechnical Assurance & ImplementationCommented:
Zaheer IqbalTechnical Assurance & ImplementationCommented:
browswAuthor Commented:
Sorry should've added I've disabled HTTP compression and set up and HTTP protocol definition without any filters on it.
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Zaheer IqbalTechnical Assurance & ImplementationCommented:
Do you have any other content filter web proxy that maybe blocking it?
browswAuthor Commented:
No, HTTP filtering and malware detection is disabled
browswAuthor Commented:
Solution was to follow instructions here:
(create an allow rule for HTTP with no filters, followed by a deny rule for HTTP with filters for that device.

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browswAuthor Commented:
Found solution elsewhere - posted as answer
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