Error Code : BLZBNTBNA0000000C
There is a link that came with error pop-up, followed it and downloaded “DigiCert Assured ID Root CA” and followed instructions- did not work. Re-downloaded client set-up, and when accessed, comes up with the same error message, saying that the digital signature of the Battle . net Agent cannot be verified…
Hello Orangery 
Sorry to hear you’re running into this error code… if re-downloading the DigiCert certificate didn’t work, let’s try doing a deep-clean of the Battle.net cached files, which will let it re-authenticate everything it needs.
In addition to the ProgramData folder mentioned in that article, let’s also clear out Battle.net, Blizzard, and Blizzard Entertainment folders in these locations (hit the Windows key and type the string to access each one):
%appdata%
%localappdata%
%temp%
Re-run the Battle.net app afterwards and let it go through its paces!
Best of luck in your adventures 
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