Description mentioning about "Files are infected" was detected.
It is due to your antivirus on file proxy / repository was performing scanning when backup. Example: Windows Defender.
You need to create a whitelist or exclusion. Additional exclusion, please refer to https://www.veeam.com/kb1999
Antivirus software may significantly slow down file share backup jobs. To improve performance, we recommend you exclude the c:\Program Files (x86)\Veeam\Backup Transport\x64\VeeamAgent.exe process from the antivirus scan on machines running the file backup proxy and backup repository roles.