Veeam will report when an “Item may have a virus reported by the virus scanner plug-in.” This warning is caused when Veeam for M365 attempts to backup an item that the built-in Microsoft 365 virus protection has flagged as possibly being infected. If you were to attempt to download the file that is reporting the malware/virus you should be met with a “This file is compromised” message:
It is recommended that you get your security team involved to investigate if the item has been compromised. Additionally, removing the infected item will clear up the warning in Veeam for M365.
Additionally, information on Microsoft built-in virus protection can be found HERE.
Source: https://www.managecast.com/veeam-for-microsoft-365-item-may-have-a-virus-reported-by-the-virus-scanner-plug-in-warning/