Do you still have the old PC? That is, can you still recreate a copy of the 2021 profile? If the copy was created while Thunderbird was running, some files may not have been included. A screenshot of the Mail folder (for POP accounts) and Imapmail folder may show what is or isn't there.

Dear David, unfortunately not, the TB profile has been copied from the hdd of the old pc. Taken directly from the directory users/.../appdata/roaming/thunderbird. I'm afraid that's the only copy I've got. I tried by just copying the profile into my new corresponding directory und alternatively by importing that profile via the TB function. The result ist the same: TB asks for the password, the password is correct, but all there is to be seen is the adress-book.

I am not understanding. Password for an account? or a Primary password? Did you copy the Thunderbird folder, overlaying the default thunderbird folder? Can you show a screenshot of the profile's Mail and Imapmail folders? That may be helpful.

Afraid I got lost with your question at that point. I transferred the whole folder named "Thunderbird" into the directory ../../roaming not by overlaying but by replacement. That has always been working fine for transferring email-accounts, stored emails and adressbooks for ten years or more. As far as I understand it - but maybe I'm missing some point in this respect - I can see the folders You mentioned above (mail and imap) only inside thunderbird. That is unfortunately the problem I have: no accounts, no mails, only the corresponding adresses. So what could I make a screenshot of?

overlaying and replacing were just semantics; I did not mean to confuse. If those mail folders are empty then the copy did not take place properly. I don't know what you mean by "only the corresponding addresses"? Do you still have access to the old profile?