Significant bug, it appears:
Ever since downloading the new server update (0.9.16.0), new additions to my Plex library are not recognized at all. I've spent several hours debugging and confirming details - and have been able to determine that the files I'm attempting to add are correct, metadata is correct, hierarchy naming is correct, permissions on the source files are correct, etc. However, the server simply doesn't "see" them, no matter how many times I pursue a rescan of the area holding this content.
Due to the timing (just after performing the server update which was offered), I surmised that the server itself might be at fault. Since the older version of the server (0.9.15.6) was still available from https://plex.tv/downloads, I downloaded the older version, stopped the server, and replaced it with this older implementation.
The problems disappeared immediately.
100% reproducible: If I restore to the newer server, new additions to the library are disallowed; if I revert to the older server, all scans/updates proceed as expected. In addition, note that similar issues are being reported by others - see:
https://forums.plex.tv/discussion/209441/anyone-else-having-trouble-adding-new-content
(which I've added similar comments to, as well)
Is this something which the Plex development team is not yet aware of? I'll be happy to provide additional details, as needed. But, I'm really hopeful that the problem can be addressed ASAP - and that in the interim, the new (problematic) server can be suppressed from being offered to users, and a support article written regarding this issue.