[GenesisDH]

Same issue here with Kodi 18.4 and Netflix addon 0.15.3 at a Nvidia Shield...

I have the same setup.  Just added the netflix addon today, enter creds, and boom.  Error.  Failed validation key exchange. 
Does this have something to do with the ESN ID?

Partially. When a Shield is updated to Android Pie/9 (Shield Experience 8.0), it receives a new Netflix EID. Also there’s a known Shield-specific problem with Widevine/DRM certificates and server-side verification that may be affecting all Shield users. There’s been bug reports sent to the maintainers of this plugin, the input-stream adaptive addon, as well as Nvidia as it also affects some non-Kodi apps.

NVidia hopes to have an update ready soon that hopefully fixes the issue (there is a hot fix available; it isn’t certified so it may break other stuff). Once that official update happens, either the addon will work as expected or at least the addon developers can use the new data points to work on a solution.