L2TP IPSec VPN failures after upgrade to MacOS Big Sur

After upgrading to Big Sur (from up-to-date Caralina) this week I discovered half of my VPN connections to remote sites are no longer working. The error is the "L2TP-VPN server did not respond" .. however pretty sure you get the same error if the shared secret is mis-matched.

All VPN connections continue to work as before from Windows 10.

Target VPN servers are Draytek Vigor ADSL routers

I've tried manually re-created the VPN configuration on the Mac, but no luck.

The only thing I can think of is an incompatibility between certain models of Draytek router with BigSur. I've seen people reporting problems with Big Sur beta releases but no fixes.

Anyone else with similar?

Thanks