One of the examples of the above error message is as follows:
User is attempting to access the resources internal to the network. User accesses the NetScaler Gateway and with the means of /cvpn gets connected to OWA or mail exchange. 
Above error message is observed once the user enters the user credentials on NetScaler Gateway and at the time page is redirecting to OWA or email exchange server, the below error message is observed:
"Http / 1.1 Gateway Timeout Unable to find the requested server or DNS Error."

Created A-DNS record of the internal URL on the NetScaler to avoid contacting the internal/external DNS server.
Reference URL: https://docs.citrix.com/en-us/netscaler/11/traffic-management/dns/configure-dns-resource-records/create-address-records.html

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Problem Cause

This would be the case of DNS record not found for the internal network resource, NetScaler is trying to contact the DNS servers which are externally/internally hosted and no response from DNS server is observed