Error 0x800b0109 connecting to VPNSSL

Hello,

I get an error 0x800b0109 when connecting to the VPNSSL using client version 4.0.5.0 from Windows 10.

The USG Flex 200 is up to date running 5.37(ABUI.1) and not showing me newer versions when checking.

vcredist 2015 are installed for both x86 and x64, the helper service is running, no AV software are installed at all, Windows firewall is disabled, SecuExtender local root cert (which is a security flaw) is present. I even exported the default cert from the USG to import it to Windows' trusted authorities store which didn't help.

On the USG side, no errors are logged, the USG confirms the credentials were correct.

Copy of logs available in #417177

The logs are not useful to resolve the issue as it claims an issue related to the root certificate while the USG self signed cert was added as root in the certificate store.

Local Zyxel support office were not able to reproduce the issue, but I was able to reproduce it on multiple computers, including computers belonging to different organizations. I'm open to using a version with more verbose logging to pinpoint the issue. OpenVPN on the same computer works correctly, which means the problem isn't upstream to OpenVPN but specific to Zyxel's implementation.