PowerPoint Web App 415 Unsupported Media Type

Question

Hi,
i had successfully deployed and configured external access via TMG to SharePoint 2013 with Office Web Apps 2013. Everything works fine, except PowerPoint Web App, when the farm is accessed externally. User becomes following error message:
Either you´ve lost network connectivity or our server is too busy to handle your request. Please check your network connection and try again later. 
PowerPoint Web App is working fine, when accessing SharePoint form local network or through VPN.
Ive searched TMG Logs and found this 415 ERROR:
Allowed Connection
Log type: Web Proxy (Reverse)
Status: 415 Unsupported Media Type
Rule: (SERVER FQDN)
Source: Internal ()
Destination: Local Host ()
Request: POST http://(SERVER FQDN)/p/ppt/view.https.svc/jsonNtlm/GetPresentation
Filter information: Req ID: 0be8d6bf; Compression: client=Yes, server=No, compress rate=0% decompress rate=0% ; FBA cookie: exists=yes, valid=yes, updated=no, logged off=no, client type=public, user activity=yes
Protocol: https
User: domain\name
Additional information
Client agent: Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko
Object source: Internet (Source is the Internet. Object was added to the cache.)
Cache info: 0x40020000 (Response includes the CACHE-CONTROL: PRIVATE header. Response should not be cached.)
Processing time: 16 MIME type:
Thank you for answers! 
Regards
Lubomir

Answer 1

I'm going to move this to the TMG forum as it is more of a TMG-related issue (clearly WAC works fine when not taking TMG int account).
Trevor Seward
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