It took some time to figure out the issue, but could finally resolve it.
The VA is configured with Proxy. So, even after opening the firewall and ports, the requests were flowing via the Proxy server instead of the VA. As a resolution, I added the Proxy server IP in the proxy.yaml file, and aggregation started working like charm! 
Thanks to all the community members for help and inputs. In case you have additional questions on this issue, ping me.
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