Can't set up IMAP account in Thunderbird

I've had a POP3 account for years, with no problem. On a second computer I had a second TB installation that used to get its emails from a folder on the main computer. This suited our purposes very well. The second user could access all my e-mails, and her sent messages also appeared on my computer. Alas I updated TB, and have lost this capability. Ideally I'd go back to it, but the only way forward seems to be to set up an IMAP account. I have tried this, following the instructions on the Mozilla web site. It's all very easy, but I end up with a message: "Thunderbird failed to find the settings for your email server". I have of course checked that the settings are correct, but I can't get past this point. I have tried disabling my Internet Security (Bitdefender) but to no avail. What should I do next, please? I'd go back to an earlier version of TB if that would enable my older method of working.