Current User: '%1' Client Access server "%2" tried to proxy Web management traffic to Client Access server "%3". This failed ...

Current User: '%1'
Client Access server "%2" tried to proxy Web management traffic to Client Access server "%3". This failed because one of the following  configuration problems was encountered:

1. "%3" 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 Web management virtual directory that this proxy traffic is going to. You can set that parameter using the Set-EcpVirtualDirectory 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 Internet Services Manager on the Client Access server "%3".

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