The most common reasons may caused SSL VPN issues

Zyxel_Stanley

The Following factors will lead to SSL VPN establishing failure.

(1) The virtual interface named "TAP-Windows Adapter V9" not installed on PC successfully.

(2) DNS is not configured properly on the PC

(3) SSL VPN server port number not configured properly on SecuExtender.

(4) SSL VPN users are not added into SSL VPN member.

(5) “admin” type users are not allowed to be used as SSL VPN login users

(6) Ethernet Interface IP address overlapped to default SSL VPN interface IP(192.168.200.1) 

The Following factors will lead to SSL VPN disconnect.

(1) The network connection is unstable between client and USG 

(2) PC wired or wireless interface status has changed. (IP change/link up/link down/PPPoE reconnect/ DDNS can’t resolve IP/WiFi Roaming/ WiFi signal is unstable)

(3) USG Interface status has changed. (IP change/ PPPoE reconnect/ link up/ link down)

(4) PC behind NAT router, and NAT router IP changed.

(5) PC routing table is instable or been changed.

(6) On USG, SSL VPN setting has changed.