The incremental С2С backup of SharePoint site finishes with a warning:
Failed to process change event Item/Add.
Microsoft 365 is capable of detecting viruses in files that users upload to SharePoint Online. Files may be scanned for viruses after they are uploaded. If a file is found to be infected, a property is set so that users can't download or sync the file. See more info in Virus detection in SharePoint Online
Warning "Failed to process change event Item/Add" may appear when C2C backup tries to backup the infected file (note that other issues may cause this warning as well).
To eliminate the warning remove the infected file from SharePoint site.