After upgrading search heads from Splunk 6.2.x to 6.3.x, why am I having proxy issues with my indexer cluster with error "browser is not supported"?

I've had Splunk 6.2.1 running for a while and Search Heads were accessible from Apache proxy and this is how it is configured. ProxyPass /splunk http://FQDN:8000 ProxyPassReverse /splunk http://FQDN:8000 ProxyPass /en-GB http://FQDN:8000/en-GB ProxyPassReverse /en-GB http://FQDN:8000/en-GB ProxyPass /en-US http://FQDN:8000/en-US ProxyPassReverse /en-US http://FQDN:8000/en-US This works with 6.2.x, but the moment server is upgraded to version 6.3.x, it stops working and I get error "browser is not supported". I added root_endpoint = /splunk in web.conf under system/local, but that didn't resolve the issue. Search heads UI is accessible using FQDN:8000, so Splunk is working as expected with 6.3.3. I've upgraded my indexers and cluster master and I've already configured multisite clustering on 6.3.3. But if I leave my Search Heads clustering to 6.2.x, then I can't get search heads connected to the cluster master because 6.2.x doesn't recognize multisite clustering, so I'm in a limbo. PLEASE help me out. I'm desperate here and I need all this up and running before Friday. Thank you