[Betuel]

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. 

Today my shield updated to 8.0.1, previously i had the "Failed validation key exchange" error but now is fixed with this update, the only video streams available were those up to 720p with HEVC and 1080p with VP9 profile but after manual Netflix ESN came all 1080p to 4K streams. But the plugin can not play 4K streams.(black screen)