The remote server returned an error: (403) Forbidden

The remote server returned an error: (403) Forbidden

Can anyone provide advice on how I can resolve the following errors on my distribution server that is preventing Agents from connecting to it:

Distribution Server Connectivity Info :
DistributionLastAccessName, #SERVER-NAME#, 8021, http, Failed, The remote server returned an error: (403) Forbidden.
DistributionServerName,  #SERVER-NAME#, 8021, http, Failed, The remote server returned an error: (403) Forbidden.
DistributionServerIPAddress, #SERVER-IP#, 8021, http, Failed, The remote server returned an error: (403) Forbidden.
DistributionServerFlatName,  #SERVER-IP#, 8021, http, Failed, The remote server returned an error: (403) Forbidden.
DistributionServerSecIPAddress,  #SERVER-NAME#, 8021, http, Failed, The remote server returned an error: (403) Forbidden.

TCP Port 8021 is unblocked in the windows firewall.

I have got a support case open however its taking frustratingly long for someone to get back to me today and so I thought I would ask here in the hope that somebody might be able to provide some useful information or advice. 

I have looked at the following article in the knowledge base -  https://www.manageengine.com/products/desktop-central/desktop-central-server-not-reachable.html
The distribution server itself appears with a green status in the Desktop Central console itself and receives the replication requests. I can also see that it has downloaded software from the main Desktop Central.


Many Thanks,

Richard

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