The message "Failed to Find FRN in Journal" indicates that the last NTFS Change Journal record that the SureSync Job in question processed could not be found in the Change Journal. When this situation occurs, SureSync has to assume that events were potentially missed and a full rescan will be triggered. The scan must complete to ensure everything is synchronized correctly. Once the scan is completed, it will resume monitoring for changes.
The NTFS Change Journal on a volume is set to a specific size and works on a first in/first out basis. If the Change Journal is small, it is possible that if a Job is not run for a long period of time that this message could occur because there have been too many events added to the Change Journal and it caused the record the Job last processed to be purged.
SureSync has logic to increase the size of the NTFS Change Journal on a system if it feels the size is too small. You can see the size of the Change Journal on each root path by looking at a Jog Log record using the Log Viewer and selecting "Trace and All Detail" from the "Filter Details" button on the Ribbon Bar. At the start of the trace, there will be an entry for each path that tells you how large the Change Journal is and an approximation of how many days of history that represents.
For example:
Monday, June 12, 2023 7:56:49 AM
(Trace)
Message: Change Journal Started on [spitest-win2k12]C:, Journal Area Size=400.3MB, Bytes to Read=0B, Maximum Size=394.0MB , Age of oldest entry=28 days
Path: [spitest-win2k12]C:\
Schedule Name: Setups: Publish MFT to Test Machines
Job Name: Setups: Publish MFT to Test Machines
While it should not be necessary, you can manually increase the size of the Change Journal on a machine using Microsoft's fsutil usn command documented at https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/fsutil-usn.