Office Web Apps Integration with Exchange 2013

Hello,

I'm using Exchange 2013 CU1 and Office Web Apps 2013 with latest KB both on Windows Server 2012.

Word and Excel documents viewing is working when I use the internal URL of Outlook Web App.

I have followed this article http://technet.microsoft.com/us-us/library/jj204665.aspx on how to publish Office Web Apps with TMG. But when I test from the Internet, I got this WOPI error:

Could not contact WOPI End Point. Error details - 'Unauthorized url - https://webmail.mydomain.com:443/owa/vrivoire%40mydomain.com/wopi/files/@/owaatt?owaatt=tAFBQU1rQURNd1lqazRPVGc1TFdKaVpUUXROR0kxTmkxaVpEQXlMVFZpTW1VNU1UVXlZMk16TWdCR0FBQUFBQUFwbzRCVlFmWTVTYnIrdlBXUWZrMlBCd0FrbVlDRFFCTEZUWTVodVNCc0hYV3pBQUFBUnJZREFBQ2pEZHFxV0RsTFFKQ2RJcGgrOHhhK0FBQXFtUzBmQUFBQkVnQVFBR1NTNVNnaDEyTkRsUmZ1VkE0dk53dz1ZM7pLbRTQCAEBAUB5ZmhXbDl4eXlubWtpWnBEdVkvUTVMN2JjeXpPQXVrRUVla0RkSVZnT3ptMTFYM256UXNyMUhJenloUElSZGND&access_token=Xu84fAmdWccVm6wpLIeYAomd449o4wpbkwQ9uDogXFn3eFBgbcPlaKIh1G9fLmfiReeKml5NOcrE6FlnKbiipQ%3D%3D&access_token_ttl=0'.

I'm not using a wildcard certificate on Office Web Apps, this certificate has been generated from my private AC. 

No drop on the TMG.

Any idea?

Regards,