Error trying to validate Networker server credentials

what NW server version are you using that you keep on using older NW BMR iso's? As those iso's also get fixes applied to them (however sadly barely any mentioning of them with each new nw version alas).

the fact that you set the date to before 1-1-2021 already shows you are using an old bmr iso version as that 1st of Jan 2021 date issue causing the java applet not even to be started, is already fixed in later bmr versions.

Did you even try with the latest nw19.10 or nw19.9 BMR iso's? We recently had some issue with nw19.6 and 19.7 bmr iso's which where gone with using the latest and greatest nw19.9 and 19.10 bmr iso's.

As you state being aware of KB 000196593 (when the backup server is running on linux), what did you do to possibly mitigate against that issue, as we tend to set the date of the BMR client to at least one whole day earlier than the current date/time. Maybe even two days to be on the safe site, to prevent the issue where a client nsr peer certificate might be in the future (without alas having a simple way to even see information about the bmr client cert or even if you can show the cert, you would have to decode the cert to even read it (as it is only encoded but not encrypted) to see from when to when the cert is actually valid, but nsradmin is not even part of a bmr client to read local nw resource files on a client or normally interact with those resources).

Considering this post is from March, chances are that the issue has been resolved already.
Nevertheless, here you have my two cents:

If you have not done it as yet, you may want to add a client record to the "User Groups / Application Administrators"

User Group Properties - Application Administrators
Users: 
[ Something like this ]
user=system,host=<client name>

Cheers,
Enrique
<Private data removed from public view. DELL-Admin>

Alas Dell does not seem to state that much about what - if anything - might be fixed in those Windows BMR iso's. There have been various issues, but recently best results where with very recent bmr iso's. Since nw19.4 different behavior (using only nsrauth and no longer oldauth) and some timing issues with nsr peer certificates that might be lying in the future ever so slightly, thus the linux nw server objecting it, without giving any insights why? If it would state the certificate would be in the future that migt be a giveaway? Hence setting the bmr vm time (either in its bios setting or once booted from bmr iso) a day or two earlier prevented that issue (it also seemed that the timezone was not adhered to, hence setting it two days earlier than actual time, worked around that issue as well).

Had various issues along the way, but the latest nw19.9 and nw19.10 bmr iso's addressed these, as some people were even using ancient nw18.2 bmr iso's (we had a Dell provided fixed bmr one, that did not have the year 1/1/2021 date issue, needing to set the time of the bmr client to before that date as otherwise the java applet bmr gui did not start).

So even if you run a nw server that is lower than those versions, at times also using a higher bmr iso can address issues. But has been way too much trial&error really... simply due to the lack of actually clearly reporting what fix is actually in which bmr iso?