Most likely this issue:
Issue 2093 – Failure when starting parallel FTP background transfers

That's strange, as I do not see any relevant change in 5.20.2 that might have caused this. Maybe except for a possible bug in OpenSSL.
Anyway, I'm sending you an email with links to several nightly builds between 5.20.1 and 5.20.2. Please test which of them have the problem.

Yes.

Thanks. So is the 5.20.2 beta the oldest/first version that crashes?

Here it is. The first past of the log file is from the version 5.20.2 beta, it crashes. Starting at cca 13:18 is the log from the old version 5.19.6, that works just fine.
I hope it helps. Thanks you!
Jan

Thanks. Would you be able to find out what version has introduced the problem? You can download all past versions of WinSCP from:
https://sourceforge.net/projects/winscp/files/WinSCP/
If you manage to find out, please post session log files both from the last working and the first non-working version.

Thank you for your reply. I downgraded version 5.19.6 (12002 2022-02-22) and it works just fine. If it would help you to find out why it happened, I'll update to newest version again and try to send the log.

What did you update in April 2022? WinSCP? What version were you using before? Does downgrading to that version resolve the problem? Can you post a session log file?

FTP transfer stops unexpectedly while uploading large numbers of small files. This problem occurs since update in April, 2022. Before that it worked without any problem. When I'm uploading 2400 files (total size = 66 MB). WinSCP uploads about a hundred files and crashes. When I'm uploading just couple of files, it works as expected.
WinSCP is installed on Windows 10, server hosted is on Wedos.
Any idea what might souse this problem?
Thanks!