Since this problem occurs in Firefox (which only needs to activate DRM playback) and Chromium (which requires an AUR add-on) both, I am not sure why this was moved to the AUR thread.
Add/Remove programs shows widevine 1:188.8.131.520-1 is installed. Uninstalling and reinstalling has no effect.
AUR shows widevine 1:184.108.40.2060-1 as the latest release and there are no notes there about my issue.
When I start Netflix in chromium, it goes to code M7702-1003 which says "Missing Component". When I go to chrome://components to look for updating the WidevineCdn component there is none listed.
Once again this happens on Firefox also which does not require an AUR add-on. On Firefox the movie just stalls immediately.
Edit: My HTPC Cinnamon with FF 57.0.4 plays Netflix, but my daily drive Xfce (and three others) also with FF 57.0.4 will not. The only difference I note is the HTPC has AMD video and the others Intel or nVidia.