Fair enough, there’s a bunch of issues at play here. I’ll list all the ones that got me in no particular order.
1: The issue that triggered your bug report, NordVPN changed the login policy for manually configured VPN connections seemingly without communicating this. I never got any notice from them about the change, but I know they know how to contact me because I get marketing messages all the time. Not a SFOS bug in the end, although the bug report did lead to a solution, so I’m glad you filed it.
2: This issue is (probably) just me . . . I can’t seem to type the 48 random chars of uppers, lowers and letters 100% accurately and verify their accuracy in the VPN credentials dialog under ideal conditions (have my glasses, seated indoors, no distractions). This is probably not an issue for users (so far) untouched by the ravages of time.
3: (This partly addresses your “plain text file” question) The VPN credentials dialog is modal(?)* and thus a PITA, so it’s impossible to copy/pasta two strings into the two fields. I could deal with that when the NordVPN username was my email – I could remember and check the accuracy of that at least – by copying the randomish password from a password manager first, then and slamming it into the password field as soon as I opened the credentials dialog. (I think I remember seeing a bug report/feature request about improving the dialog somewhere, but can’t seem to find it now.)
4: (Another reason for the plain text file) SFOS VPN password amnesia problem. The one in this bug report: VPN credentials gone on restart 3.4 on XA2. Supposedly this is now fixed, but I’m not 100% sure it’s 100% solved.
5: NordVPN servers periodically go away. AFAIK, there is no schedule for this or notification to users ahead of time. This is not a SFOS bug either, except that login failure due to a missing server just causes the VPN to report status “Connecting” and pulse the active indicator forever, so the cause of failure isn’t readily apparent to the user. I’ve never had sufficient patience to find if this ever times out and fails.
To sum up: I’ve got it working with the new NordVPN credentials policy on freshly downloaded server configs, and I’ve purged the expired servers from my VPN settings that were causing additional, unrelated problems complicating the solution for me.