SSL VPN Plus connection failing from a Ubuntu 18.04 client

Products

VMware NSX

Issue/Introduction

Cannot connect to SSL VPN Plus service running on an ESG from an Ubuntu client
SSL VPN connections from this Ubuntu machine are failing when using the GUI or the CLI without any error message.

In Ubuntu syslog:
Oct 5 05:50:48 ubuntu naclient[9817]: info::Populating profile info, gateway ip, port, interface and certificate info from config file.
Oct 5 05:50:48 ubuntu naclient[9817]: debug::Gateway ip and port : 10.123.20.34:4443
Oct 5 05:50:48 ubuntu naclient[9817]: debug::Reading mac address and client version from /opt/sslvpn-plus/naclient/naclient_install.conf
Oct 5 05:50:48 ubuntu naclient[9817]: debug::Client version : 6400
Oct 5 05:50:48 ubuntu naclient[9817]: debug::MAC address : 00:FF:84:1C:73:9F
Oct 5 05:50:48 ubuntu naclient[9817]: debug::MAC0 : 0
Oct 5 05:50:48 ubuntu naclient[9817]: debug::MAC1 : ff
Oct 5 05:50:48 ubuntu naclient[9817]: debug::MAC2 : 84
Oct 5 05:50:48 ubuntu naclient[9817]: debug::MAC3 : 1c
Oct 5 05:50:48 ubuntu naclient[9817]: debug::MAC4 : 73
Oct 5 05:50:48 ubuntu naclient[9817]: debug::MAC5 : 9f
Oct 5 05:50:49 ubuntu naclient[9817]: info::Sent command to server daemon to start SSLVPN process.
Oct 5 05:50:49 ubuntu naclient[9817]: debug::Authentication required from gateway side.
Oct 5 05:50:49 ubuntu naclient[9817]: error::No error message. Invalid error code : 32512
Oct 5 05:50:52 ubuntu naclient[9851]: info::Populating profile info, gateway ip, port, interface and certificate info from config file.
Oct 5 05:50:52 ubuntu naclient[9851]: debug::Gateway ip and port : 10.123.20.34:4443
Oct 5 05:50:52 ubuntu naclient[9851]: debug::Reading mac address and client version from /opt/sslvpn-plus/naclient/naclient_install.conf
Oct 5 05:50:52 ubuntu naclient[9851]: debug::Client version : 6400
Oct 5 05:50:52 ubuntu naclient[9851]: debug::MAC address : 00:FF:84:##:##:##
Oct 5 05:50:52 ubuntu naclient[9851]: debug::MAC0 : 0
Oct 5 05:50:52 ubuntu naclient[9851]: debug::MAC1 : ff
Oct 5 05:50:52 ubuntu naclient[9851]: debug::MAC2 : 84
Oct 5 05:50:52 ubuntu naclient[9851]: debug::MAC3 : 1c
Oct 5 05:50:52 ubuntu naclient[9851]: debug::MAC4 : 73
Oct 5 05:50:52 ubuntu naclient[9851]: debug::MAC5 : 9f
Oct 5 05:50:52 ubuntu naclient[9851]: info::Sent command to server daemon to start SSLVPN process.
Oct 5 05:50:52 ubuntu naclient[9851]: debug::Authentication required from gateway side.
Oct 5 05:50:53 ubuntu naclient[9851]: error::No error message. Invalid error code : 32512
Oct 5 05:50:55 ubuntu naclient_poll[8693]: poll.c:231 cli_status ret 4

In the Edge logs we see a "resource temporarily unavailable":
2020-10-05T12:50:50+00:00 NSX-edge-3-1 sslvpn[]: [default]: [user.debug] DEBUG ::SSL VPN-Plus CLIENT : Received command = 16, protocol=9
2020-10-05T12:50:50+00:00 NSX-edge-3-1 sslvpn[]: [default]: [user.info] INFO ::SSL VPN-Plus CLIENT : Connected from Linux client: version=6400
2020-10-05T12:50:50+00:00 NSX-edge-3-1 sslvpn[]: [default]: [user.info] INFO ::SSL VPN-Plus CLIENT : Sending authentication request to PHAT client..
2020-10-05T12:50:50+00:00 NSX-edge-3-1 sslvpn[]: [default]: [user.debug] DEBUG ::SSL VPN-Plus CLIENT : Received command = 10, protocol=9
2020-10-05T12:50:50+00:00 NSX-edge-3-1 sslvpn[]: [default]: [user.debug] DEBUG ::SSL VPN-Plus AUTHENTICATION : Tunnel ID 26, received authentication request for user test.
2020-10-05T12:50:50+00:00 NSX-edge-3-1 sslvpn[]: [default]: [user.debug] DEBUG ::SSL VPN-Plus AUTHENTICATION : PHAT client IP address for newly added session = 192.168.182.128
2020-10-05T12:50:50+00:00 NSX-edge-3-1 sslvpn[]: [default]: [user.notice] NOTICE ::SSL VPN-Plus CLIENT : PHAT login success for user test with IP address 192.168.182.128 and virtual IP address 172.16.60.100
2020-10-05T12:50:50+00:00 NSX-edge-3-1 sslvpn[]: [default]: [user.debug] DEBUG ::SSL VPN-Plus CLIENT : 268 bytes pending to write to tunnel 26: state=--
2020-10-05T12:50:50+00:00 NSX-edge-3-1 sslvpn[]: [default]: [user.err] ERROR ::SSL VPN-Plus SERVER : tunnid=26, read failed with error: 0, Resource temporarily unavailable
2020-10-05T12:50:50+00:00 NSX-edge-3-1 sslvpn[]: [default]: [user.debug] DEBUG ::SSL VPN-Plus CLIENT : Request to free tunnel : 26
2020-10-05T12:50:50+00:00 NSX-edge-3-1 sslvpn[]: [default]: [user.notice] NOTICE ::SSL VPN-Plus CLIENT : PHAT session logout success for user test with IP address 192.168.182.128 and virtual IP address 172.16.60.100

Environment

VMware NSX Data Center for vSphere 6.4.x

Resolution

This is a known issue affecting VMware NSX Data Center for vSphere versions below 6.4.6.

Workaround:
It is suggested to upgrade NSX-V 6.4.6 version onwards with "Ubuntu 18.04" when using SSLVPN plus client.