The Neftex Download command reports an error when trying to send the users Neftex login details: “The underlying connection was closed: An unexpected error occurred on a send”.

The issue was caused by Halliburton removing support for the TLS 1.0 protocol (TLS is a cryptographic protocol for establishing a secure communication between two systems) on the Neftex webserver. This was recommended due to vulnerabilities being identified in the SSL/TLS 1.0. 

The error is resolved in Player 4.3 SP2 (Build 6799). The fix was to use a minimum of TLS 1.2 when communicating with Neftex servers. This fix was also applied to IHS Download command. As Esri do not support TLS 1.2 on ArcMap 10.2, the fix only applies to ArcMap 10.3-10.6 when Microsoft .Net 4.0 is installed on the machine. For more info on Esri's support of TLS: TLS_Guidance.pdf.