Thanks. But what about the log from the other client?

Also, in this case the disconnect came after almost an hour of inactivity. No surprise there. If you need to keep the connection alive all the time, try enabling keepalives:
https://winscp.net/eng/docs/ui_login_connection

Here:

Yes, but the location does not exist

My post above links to a documentation that shows how to configure logging, including log file location.

Yes, but where can I find this file?

@Lukas31: Please attach it to a post here.

And how can I send them the log?

In WinSCP: https://winscp.net/eng/docs/ui_pref_logging
I do not know what other SFTP client did you try, so I cannot help with it.

If only I knew where to find them

Please post logs from both.

Yes, unfortunately it also occurs there, but not as often and persistently as with WinSCP.

Thanks for your report. Do you have any reason to believe that it's WinSCP's problem? Did you try other SFTP clients? Don't they lose the connection too?

The connection to the server is unstable. WinSCP repeatedly loses the connection when opening files or switching directories. Disconnects occur at irregular intervals, ranging from approximately 5 seconds up to 10 minutes.