Chaper thumbnails suddenly not being generated

[NewPlaza]

That should of been fixed in version 1.15.4.919

Plex Media Server

Plex Media Server 1.15.4.919 is now available to Plex Pass users in the Beta update channel NEW: (DVR) Ability to give Home Users DVR access. (#8794) (Playback) Assume unknown audio to be foreign if the preferred audio language is not english (#6126) (Streaming Brain) Preference to allow automatically terminating playbacks which have been paused for an hour (#9700) (Subtitles) Auto select forced subtitles when not listening to foreign audio (#6126) When transcoding for supported clients, auto…

I guess in the interim, you can analyze your videos to build the chapters/chapter thumbnails.
https://support.plex.tv/articles/200289336-analyze-media/