Thunderbird does not work if connected to NordVPN unless Email Shield is switched off

As of about 3 weeks ago, I get the following error when using Thunderbird to access my various email systems if connected to NordVPN:

could not connect to server xxxx; the connection was refused

If I switch the Email Shield off, there isn’t a problem when connected to NordVPN.

I have repaired AVG Internet Security as suggested in a previous post but that doesn’t help.