There is an issue with Plex Web for Chrome playing YouTube Watch Later videos. They just sit there loading forever.
There was an issue with PMS for Windows v1.5.2 being unable to transcode YouTube videos, but that issue has been resolved in PMS for Windows v1.5.6 and later. I know this issues is resolved because the Plex app for Roku must trancode some YouTube Watch Later videos and they play and transcode fine on the Plex app for Roku using PMS for Windows v1.5.6 or later.
The same YouTube Watch Later videos will direct play on the iOS app and transcode on the Roku app. But I have tried playing these YouTube videos in Plex Web v3.2.1, v3.4.1, and v3.6 in Chrome and they just sit there loading forever.
I even tried disabling Direct Play under Settings > Plex Web > Player, but they still will not play.