The incremental С2С backup of SharePoint site finishes with warning:
Failed to process change event Item/Add.
Office 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 backups tries to backup infected file.
To eliminate the warning remove infected file from SharePoint site.