does this involve a gmail or outlook,com mail account?

yes. In fact it is with two gmail accounts. The emails enter without problem, but for salor they take a long time and there is always an error of copying to the sent folder. I have to close the window, finally the email is sent and an error window continues to appear.

If your account is IMAP and I suspect it is. Right click the account in the folder pane. Select settings. In copies and folders for the account change the option to tell Thunderbird to not do anything with copies of sent mail. Most certainly not save a copy in the sent folder

Both Gmail and Outlook,com place emails sent through their outgoing server in the sent folder. With Thunderbird doing it as well there is a conflict and an error because two adds are happening basically at the same time.