Resuming transfers

I read this: https://winscp.net/eng/docs/resume

I'm hosting a server (Ubuntu 22.04) and a friend (on Windows) sometimes gets terrible transfer rates. When his transfer is interrupted and for "reasons" he is unable to resume in that session, he is unable to resume the next day and has to start over. He and I both use the Explorer style UI.

Is the resume feature only available in the Commander interface?

Interface does not matter.
Please post session log file documenting the "unable to resume".

He doesn't have a log. If it happens again I'll try to get him set up to keep logs.