You may encounter issues while using protonvpn on Ubuntu. I ended up multiple times without any way to reach servers. Pinging IP works, but not DNS resolution. It was related to a ProtonVPN issue.

When connecting to ProtonVPN, some connections are created.

nmcli connection show --active # to list all active connections
nmcli connection show # to list active and inactive connections

When you are not connected to a ProtonVPN server, there is still one active connection related to ipv6, pvpn-ipv6leak-protection.

Delete the connection:

nmcli connection delete pvpn-ipv6leak-protection

Should be fine now.

It is also causing issues while my laptop is suspended.