I have the same problem with larger files but on just one client. I tested with 300KB files and I got the 0KB filepart, I did a test with a 67KB file and got the 0KB filepart, when I did a test with a 2KB file it uploaded just fine. I am using the latest NuGet package, the version is 5.21.2.0. I had the same problem in an older version of WinSCP as well. I am using SFTP as a server, the server has plenty of drivespace, the server is running FreeFTPD and is working well with other clients uploading using the .NET WinSCP.
The SFTP server shows a disconnect right after the filepart upload which does not show as a failure.
08-24-2022 15:20:59 HOST 173-13-11-33-Michigan.hfc.comcastbusiness.net SFTP 143 is changing dir to / (C:\inetpub\wwwroot\GCRemoteTools_WaltersDimmick\AuditMedia\143\)
08-24-2022 15:20:59 HOST 173-13-11-33-Michigan.hfc.comcastbusiness.net SFTP 143 is listing /In (C:\inetpub\wwwroot\GCRemoteTools_WaltersDimmick\AuditMedia\143\In\1614_c4_0_1661262050.jpg)
08-24-2022 15:20:59 HOST 173-13-11-33-Michigan.hfc.comcastbusiness.net SFTP 143 is listing /In (C:\inetpub\wwwroot\GCRemoteTools_WaltersDimmick\AuditMedia\143\In\1614_c4_0_1661262050.jpg.filepart)
08-24-2022 15:20:59 HOST 173-13-11-33-Michigan.hfc.comcastbusiness.net SFTP 143 is uploading /In/1614_c4_0_1661262050.jpg.filepart (C:\inetpub\wwwroot\GCRemoteTools_WaltersDimmick\AuditMedia\143\In\1614_c4_0_1661262050.jpg.filepart)
08-24-2022 15:21:00 HOST 173-13-11-33-Michigan.hfc.comcastbusiness.net SFTP 143 disconnected
Let me know if you need anything else, thank you for looking into this matter, it is greatly appreciated.
Description: Logs from failed 300KB upload. There were 288 files in the folder that were about this size.
Description: Logs from failed 67KB upload.
Description: Logs from 2KB successful upload.