Is this still an issue? Around the date you asked the question 123-reg were changing over their email servers, not just a server swap but a migration to a new email service. My understanding is email accounts from the old service are being migrated to the new service. Between old servers and new servers, server settings POP, IMAP, SMTP are changing (I am unsure if the change affected user passwords).

Using a browser, can you login to 123-reg webmail directly? If so check which service you are using and check you have the correct settings for this service.

If you need to re-configure Thunderbird for a new service, you may need to re-post as a new queary, to correctly understand how to move your account, and Thunderbird settings, forward.

Simon. Thanks for your reply. Yes I saw that accounts are being migrated. However, on the phone with 123-reg, I was told to set up as generation one. (If that is the question). They have extensively tried to help me set up in TB. Yes, I can access emails on their webmail. This verifies that the password at least is correct and that the issue is within Thunderbird.