I too have had the missing metadata/posters bug when installing newer versions of Plex server on Synology (I've reverted to Version 0.9.16.2). I keep seeing this explanation on this forum:
@trumpy81 said:
If you are not logged on to plex.tv and you add new media, Plex is not able to update the database at plex.tv or locally.
I've installed the latest version of the windows server on a PC and it has no problem showing me the posters without logging on to plex.tv, whereas the Synology version won't. So, it seems to be a Synology only bug. Can anyone else confirm the same behaviour?
I'd speculate that the posters do exist but that the synology version of plex media centre just isn't serving them.
No, I don't want a Plex account and will stay at Version 0.9.16.2 if necessary.
SirThanksALot