Event ID 55 is written to the System log shortly after a system backup has begun. However, when you run chkdsk, no problems are found.
This behavior occurs when the SYSTEM account permission is removed from a file or a folder on a volume. When a backup is run in this situation, Windows cannot read the file or folder because the SYSTEM account doesn't have permissions to it. Therefore, Windows marks the file or folder as corrupted. However, when chkdsk runs, it does not require permissions to check for corruption on the file, and therefore it finds no problems.
To resolve this issue, locate the file or folder that had the SYSTEM account permission removed, and add back default permissions to that file or folder.