The remote endpoint no longer recognizes this sequence. This is most likely due to an abort on the remote endpoint. The value of wsrm:Identifier is not a known Sequence identifier. The reliable session was faulted.
If upon checking those logs you find the following error, see below for the resolution.
System.Data.SqlClient.SqlException (0x80131904): Cannot open backup device '\\backup_location\filename.esync.bak'. Operating system error 53(The network path was not found.).
BACKUP DATABASE is terminating abnormally.
The automatic sync backup location is inaccessible. Ensure the path is correct and reachable in the backup sync settings.
Backup sync settings are found in the following location.
Settings > Configuration > Synchronization > Settings