I'm not able to determine what "Fast SSL" means from the DrayTek documentation, but I did see that this setting is now disabled by default in the later 2019 releases. I'd guess perhaps it shortens the TCP retransmit timers so that it is more more "impatient", particularly when negotiating the initial HTTPS connection for the VPN. It is possible that it isn't getting a fast enough response for its impatience and is retransmitting before it is supposed to based on RTT, and running afoul of the Inspection Setting "Invalid TCP Retransmission". This enforcement will basically stall the HTTPS/443 connection in this case and result in the disconnection of the VPN.
Try changing the Inspection Setting "Invalid TCP Retransmission" from Drop to Accept on the firewall and see if Fast SSL still works. I'd really not advise leaving this protection in the Accept state though, and believe that disabling Fast SSL would be a better solution as clearly whatever that feature is doing is not RFC compliant.
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