There is a bug in macOS Big Sur 11.0.1 where VPNs of type Cisco IPSec won’t work when a Network Extension is installed. This affects Little Snitch 5 (not older versions!) and many other products based on Network Extensions. The VPN will connect successfully, but no data is transferred.
The bug has been reported to Apple and will be fixed in macOS 11.1.
If you rely on this type of VPN, please consider installing Little Snitch 4.6, which is based on a Network Kernel Extension and is therefore not affected.