You are not alone. There are a number of us reporting the same issue, another user spotted that it seemed to happen after server version 1.18.3.2129, having downgraded back to that, everything is working again for me. Hopefully someone at Plex will see all these posts/logs and be able to track down the issue.
My post (including logs): Plex sync stuck at 50% (waiting) on all devices
Post identifying last working version: Sync stuck on "Waiting to download"
