After raising a support ticket, I've just been informed by NordVPN support that NordVPN won't work with LibreElec 11/Kodi 20.
The actual reply was:
LibreELEC 11 has been updated to Kodi 20, which uses Python 3. Unfortunately, our app on Kodi only works with Python 2.7. Also, LibreELEC 10.0.4 uses Kodi 19.5, which also has Python 3 now. The reason why it may have worked before could have been that the Kodi versions used before were older.
This is depite the fact that I've been running v. 10 (Kodi 19) with no problems for some time!
Can anyone recommend a VPN that runs OK on the latest version.