Configuring L2TP VPN to use with iOS 14 and macOS Big Sur
I was having this same problem after updating,
change sha2-truncbug=yes > sha2-truncbug=no
Using IPsec and xl2tpd for L2TP over IPsec server. The client is macOS Big Sur Version 11.0.1
Unfortunately, I have not figured out what that configuration key does, and it's not documented anywhere so I cant give you a good explanation why this works.
Make sure to do a systemctl restart ipsec after updating the config file.
Edit: Found it documented on github