Adding email accounts failing on new PC: thank you but it is the email accounts failing to be created (locked duplicate)

Thank you for the advice on copying the Thunderbird folder, which I have done. However, what is failing is the adding of further existing email accounts. I enter the Account name, the email address and the password but I still get Authentication error. I have, after some difficulty, succeeded in adding five accounts with three different hosts. The further accounts I wish to add use two of the hosts of the successful additions.