Thanks for your solution, mate. I was very upset and frustrated before. My apologies!

No, the problem is related to VPN provider apps. As long as one is using linux defautls, (preferably OpenVPN) all works irrelewant of VPN provider's customizations.
It took me 5min to set OpenVPN with NordVPN.
There are more complicated (if needed to circumvent firewall for example) ways to set up OpenVPN.
More importantly, VPN provider will have OS related howtos or one can contact them and they should help.
I never had an isdue with OpenVPN, but I never used browser plugins or VPN provider's aps.
Main reason being that irreleva t of OS they tend to fail at some point.
There are only two reasons to disable ipv6 (second conditional):
1) privacy issues
2) DNS leak if using VPN (a - disable ipv6 in browser, b- didable ipv6 system wide)
However presence or absence of ipv6 will not affect connrctivity. If it does then the isdue is with the machine particular config)

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Maybe, but a major problem suddenly arose when
needed changing. To login to a NordVPN browser-plugin one has to input user account details; when reloading browser tabs or in the nm-applet, the credentials should be those generated in a second step one can do after logging into one's NordVPN account on-line as described in those threads.

I never used nm applet for VPN conection management so I can't tell how it works. I would never re-load browser for VPN.
KDE Plasma Network Manager handles simple ovpn file without issue (including NordVPN). Less obvious VPN configs require manual input but work anyway. Changes to ovpn file are not limited to NordVPN. This seems more common approach. I don't know what caused this though as credentials are saved in plain text.

cwizardone,

The Google Chrome NordVPN extension solution is dead simple and an easy fix.

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Works. Interesting. Tried it in Vivaldi and the nordvpn connection only works for whatever connections are made from inside of Vivaldi. Fired up two other browsers and they don't use the nordvpn connection. That is fine. The extension does make it easier to connect and disconnect.
Thanks!

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You're welcome, cwizardone! A big shout-out to leclerc78 for the solution.

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It has been quite a while since I looked at extensions, so I just checked and found the same, for all practical purposes, extension is now available for Firefox. Looks and works the same as the extension for chrome and its clones.
Thanks, again.

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You're welcome. Just installed it on FF. Thanks for the tip. Cool!

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@hitest and @cwizardone
Probably too late but final guestion/solution: why don't you guys set up wireguard (NordVPN provides config files for wireguard)?
It will solve your authentication issues. Just remember to adjust your iptables config for wg0
Maybe this will help if not now then in the future with similar problems. I would test it even just as an excersise.

Good luck

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Good morning, Aeterna! It's never too late, mate. If you feel like it you could perhaps post your solution to the Slackware documentation project. A suggestion. We appreciate your input.

https://docs.slackware.com/

Sure, I can write (very short) howto

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cwizardone,

Now that NordVPN works with Fire Fox I removed Chrome. The better to hide from the Google Over Lords. Haha.

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Learn something new everyday. I will look into WireGuard also when I have some free time, brief searches online give it praises; although, I have no issues with OpenVPN.

This is simple how to set up wireguard. If someone follow these steps successfully I will add howto to https://docs.slackware.com/
1) download wireguard config files from your VPN provider
2) wireguard accepts only 7-letter names
2a) wireguard interface name is created from config so if config name is aaaaaa0.conf then interface created will be named aaaaaa0
2b) following accepted rule rename downloaded wireguard config file to wg0.conf

3) add wireguard config file named wg0.conf to Network Manager (it is impossible as of now to directly add wireguard config file to NetworkManager)
this will add wireguard config to Network Manager

4) Now you can connect to wireguard from Network Manager
that is all

Notes:
1) if for some reason wireguard connects before ISP, you have to disconnect wireguard, connect ISP, re-connect wireguard or see Notes 5
2) testing DNS leaks:
https://ipleak.net/
https://www.perfect-privacy.com/en/tests/dns-leaktest
https://www.dnsleaktest.com/

Notes 2:
iptables changes are simple following tun0 or eth0 or wlan0. For multiple wgX interfaces use wg+

Notes 3:
default Slackware kernel has wireguard and friends enabled.

Notes 4:
check if wireguard is up and working:
Notes 5:
forgot to add: while automatic connection with wireguard is fine for desktop, it is problematic with mobile. In the case that you don't want auto
connection to wireguard run:
where wg0 is the connection's name