You may not be able to fix this problem. It's not really a WinSCP bug. Rather, it's an issue with Microsoft OneDrive.
Recently, I included the portable version of WinSCP.exe in a zipx file, along with various Excel and data files. I posted it on my Microsoft OneDrive (formerly SkyDrive). I obtained a link for the file and sent it to a friend.
When my friend clicked the link, his Browser loaded the OneDrive folder and correctly displayed an icon for the zipx file. But when he tried to download it, he got the following message.
"Unable to scan Audit_Support_Center.zipx for viruses
OneDrive is unable to scan Audit_Support_Center.zipx for viruses."
I experimented a bit more. I now think that OneDrive refuses to download ANY exe file. Probably, this is Microsoft policy, and you can't change it. But I thought you'd like to know about it.