Client Access server "%1" tried to proxy Outlook Web App traffic to Client Access server "%2". This failed because one of ...

Client Access server "%1" tried to proxy Outlook Web App traffic to Client Access server "%2". This failed because one of these configuration problems was encountered:

1. "%2" has been set to use "http://" (not using SSL) instead of "https://" (using SSL). You can modify this by setting the InternalUrl parameter of the Outlook Web App virtual directory this proxy traffic is going to. You can set that parameter using the Set-OwaVirtualDirectory cmdlet in the Exchange Management Shell.

2. The destination virtual directory returned an HTTP 403 error code. This usually means it's not configured to accept SSL access. You can change this configuration by using IIS Manager on the Client Access server "%2".

If you don't want this proxy connection to use SSL, you need to set the registry key "AllowProxyingWithoutSSL" on this Client Access server and to set the InternalUrl and SSL settings for the Outlook Web App virtual directory this proxy traffic is going to accordingly.