I had problems on LE 9.0 / generic.
inputstream from LE was a lower version than the one that was in the regular kodi.
I downloaded inputstream.rtmp from github and build this in ubuntu, the module that came from that I copied to my libreelec install, and afterwards netflix was working as it should.
so maybe a tip: upgrade your rtmp : GitHub - xbmc/inputstream.rtmp: RTMP input stream add-on for Kodi
Thanks for that!
I was able to get the Netflix addon to play videos by doing this. Took a while to figure out how to compile & install, but well worth the effort