Please go to - Explore > Firewall > Open Firewall > under “Connected to” change it to “Trusted”
If the issue persists, try to repair Avast one - Account> Settings> Troubleshoot> “Repair Avast One”
Thanks very much for the ideas- unfortunately it was already set on ‘trusted’ (toggle tried) and resetting the app didn’t solve the VPN issue. I’ll reach out to support.
Just by way of update, just for kicks after logging the issue with Support, I tried totally uninstalling, rebooting, ensuring Windows Defender was happy, then reinstalling Avast One.
Lo and behold, VPN now works perfectly to any endpoint I choose. Happy days!