Actually I wasn't able to reproduce the problem.
What version of WinSCP are you using? Can you post a screenshot of your WinSCP logging preferences and of your WinSCP workspace with the error message? What protocol are you using?

Thanks. I'll look into it.

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Note that if you want to log multiple parallel sessions, you need to make their log names unique by using some of the patterns.
See https://winscp.net/eng/docs/ui_pref_logging#session_log

If I open a second tab to connect to same server, and logging is enabled, a popup appears saying:

Error del sistema. Código: 32.
El proceso no tiene acceso al archivo porque está siendo utilizado por otro proceso

Translated to English:

System error. Code: 32.
The process does not have access to the archive, but it can be used by other processes

This popup has 2 buttons: Accept & Help
Accept closes the popup but it comes back right after, and I am stuck with the popup.
Help doesn't offer anything helpful, it says to submit message online but there are 0 search results.

So I am stuck with a popup that blocks the entire program from doing anything else.

The error makes sense, the problem is it should go away and not come back.