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

Hi,
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 (172.16.4.40:1144)
Destination: ec2-54-246-108-230.eu-west-1.compute.amazonaws.com 54.246.108.230:80
Request:  
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!
browswAsked:
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Zaheer IqbalTechnical Assurance & ImplementationCommented:
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Zaheer IqbalTechnical Assurance & ImplementationCommented:
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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?
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browswAuthor Commented:
No, HTTP filtering and malware detection is disabled
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browswAuthor Commented:
Solution was to follow instructions here:
http://blogs.technet.com/b/isablog/archive/2006/09/25/why-do-i-need-a-deny-rule-to-make-an-allow-rule-for-a-custom-protocol-work-correctly.aspx
(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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