The hint is in the folder name.
You or one of your shared users with ‘Sync’ privilege has initiated a sync of (probably) several items (or even maybe a whole library) to his/her local Plex client.
(Plex for Android, Plex for iOS and Plex for Windows/Mac can do this)
If it was you who acidentally did this, you can find (and delete) pending Sync jobs in the web app https://app.plex.tv under Settings - Server - Status - Sync
If it wasn’t you, who triggered this, your best way to find out who did it is to install Tautulli
You might also want to revoke the “Sync” privilege from your user(s) under https://app.plex.tv/desktop#!/settings/users-sharing
Click on each user name, then go to the ‘Restrictions’ tab.
Afterwards, stop and restart your server.
