So here's my scenario:
Kodi on HTPC, Emby on Server
I always let the HTPC with running Kodi go to sleep while the Server is still active. The Server itself goes to sleep some minutes after this.
The next time i wake up the HTPC while the Server is still sleeping, i get the known "Library Sync Thread has Exited" error.
Shortly after this i get the notification from the Kodi Emby addon "Server unreachable".
Imho this is a simple error in the logic how things are executed.
The addon should first check for successful connection before starting the sync thread.
If the connection is refused, then don't start the sync thread at all.
I can attach the logs in the evening if you want some, but maybe my description is enough to find the culprit?!