I suggest starting fresh, so we can follow your steps.

https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer

here is the info in brief steps:

1. ensure thunderbird is NOT running on old machine
2. enter %appdata% in search window on task bar and press Enter key
3. this should open windows file explorer in Appdata\roaming folder
4. highlight the Thunderbird folder and copy to external media
5. ensure thunderbird is installed on second computer, started once and NOT running
6. enter %appdata% in search window on task bar and press Enter key
7. you should be in Appdata\roaming folder , copy&paste the exported Thunderbird folder there. This may prompt to overwrite an existing folder of same name. allow it

#   start thunderbird and all should be there

I was very careful to follow the instructions carefully and I closed TB on both machines. Some files could not be copied on the source machine. On the target, it would not allow me to "remove" the folders, so I tried to overwrite them. That also did not work. Now the target version of TB does not work at all. Fortunately, I cloned the SSD of that machine and am restoring it now. I am giving up on this for now.

My current problem is that TB does not start up automatically when I turn the source PC on any more. Not sure why. I added a shortcut to the Start folder. Thoughts please.

It just got worse. I cannot restore the clone to my SSD on the target machine and TB is unusable. I tried deleting TB, but that did not help. Suggestions on how I can start fresh on the target machine please. It will not allow me to delete all the TB folders either. What a mess.

Please provide details. what error messages do you receive when attempting to delete folders? what errors when attempting to restore? what messages when you could not copy? what folders/files could not be copied? Thank you

The target PC (W11) TB is "broken". I cannot do anything with TB on that machine -- in other words TB does not work at all. I tried to delete TB and it said OK. But when I download TB and tried to install it, it keeps asking me for my password. I enter the password but it still does not allow me to complete the installation and continues to ask for the password. My #1 priority is to fix that and get TB working on that machine. In other words, it is very difficult for me to communicate the issues, since I have to use my phone to take a pic of the issue and then email it to my source machine and then post the info.

I guess what I am asking is how do I remove all TB remnants from that target machine (w11) and then I can download and install a fresh copy of TB? Once I do that, then I can go thru the process you suggested for moving the files from W10 PC to W11 PC.

Thanks and sorry for the confusion.

Please be clear. Are you saying that thunderbird wants a password, even before you attempt to add an account? Puzzling? Anyway, be sure that thunderbird is not runnng. - enter %appdata% in search window on task bar and press enter key - that should open the screen for appdata\roaming - locate the Thunderbird folder and delete it - now download thunderbird from thunderbird.net and install and then exit

Then, repeat the previous instructions.

I deleted TB from the W11 PC and downloaded TB as instructed. Then attempted to set it up. I entered the correct PW and then got the error shown in the attached image.

PS. Username and password are correct. There are some "remnants" of TB on the machine which I suspect are causing the error. W11 is more restrictive that prior Windows versions.

First, every day people post, saying their settings and password are correct. But there is an error, so something is wrong. try again and OMIT password. Is the domain administered by AT&T? That may be an issue. Let me know

Thanks for the idea, but no luck. How do I remove the "remnants" of a previous TB install? When I tried to move the W10 to W11, some of the files in the two folders, roaming and local, could not be removed. I suspect that is part of the problem. I guess I could go into the command prompt and remove what is left. So my question is, what folders and files to remove to completely remove TB from the W11 PC? Then I can start fresh.

https://intodns.com/websitemanage.com

It looks like this is a namecheap hosted domain sing cpanel, so most autodetercion stuff will probably fail, especially the SSL.TLS stuff because there will be a name mismatch in the certificate for the mail server.

So instead of offering the error literal lets look at the real issue and this is not going to show it.

Have you followed the instructions offered by namecheap? They appear to have been very recently updated to use the new account hub, but using either the wizard or the hub the steps are the same. https://www.namecheap.com/support/knowledgebase/article.aspx/9186/2175/private-email-account-setup-in-thunderbird/

Their advice that you can dismiss a name conflict is a little worrying as if it is dismissed at that point it will just keep reappearing until the problem is fixed and no mail will download until there is a solution offered (usually you create an exception or drop using SSL/TLS) encrypted connections

OK, I used "Restore" to get TB working on the W11 machine. So next, I will backup the system. Then I will try to move TB W10 to TB W11. So basically back to square one, but with two working machines.

I will continue tomorrow and document each step and stumbling block as I try to move W10 PC to W11 PC.

Thanks for all your help. Sorry if I wasted your time. I will be more diligent going forward. Tim