To my greatest regret the FireFTP of Firefox is not supported any more since Version 60 of FireFox. That ftp client was always easy and fast to use. OK, FileZilla and command line ftp work too, but FireFTP was more convenient. Sigh ........
I think there could be several causes for this problem. It could be because your network connection is unstable. Or your time like the date and time on your device is set wrong. It could also be because you are trying to register more devices than the device registration limit.
@lebucur
look for settings for Filezilla - maybe there is something what you miss in WinSCP.
https://docs.pycom.io/pycom_esp32/pycom_esp32/toolsandfeatures.html?highlight=filezilla settings#filezilla-settings
or use Filezilla instead