@mkx, there seems to be a lot of confusion regarding encryption - there are a few encryption protocols used on the PPP level (L2TP, SSTP, PPTP, serial PPP) to encrypt the payload before encapsulating it into the PPP transport packets, and these are unrelated to the encryption of the transport packets themselves (using IPsec or TLS), except that there is no point in using encryption at PPP level if it is provided on the transport level. So if the log at the responder (server) shows that negotiation on PPP level has started, the encryption of transport works fine.
Also configuration-wise, the
use-encryption setting on
/ppp profile row has no impact on the transport level encryption.
@masoud80kg (or rather others brought here when searching for the same issue), as @Sob wrote, 0x80fd is a compression protocol, which is also used at PPP level. So there should be a configuration option allowing to disable it completely or at least make it optional, but as you haven't found any corresponding setting on the graphcal interface, you have to search for it in configuration files.
https://manpages.ubuntu.com/manpages/ja ... ml#options should give you a hint - look for the
bsdcomp option on that page.