I'm completely stuck. Any suggestions?

"Unable to write the email to the mailbox. Make sure the fle system allows you write privileges, and you have enough disk space to copy the mailbox." After years of working fine, this message has started appearing whenever TB tries to retrieve messages. I've checked that there's enough space (there's many GB free). I've checked the authorisations of the email folders, they're all fine (Full Control in all cases). I found an answer in a forum suggesting that a rogue antivirus might be the problem, but I'm only using MS Defender. In desperation I removed the entire Roaming/Thunderbird folder, uninstalled TB, reinstalled it (version 115.12.2), created a new profile for my email address (it's a Yahoo email address, and I can retrieve the emails directly through the Yahoo Mail website with no problems). But I'm still getting the same error message. TB tries to retrieve the messages, it says 'Retrieving message 1 of 105' for several minutes, then the error message popup, and nothing appears in the Inbox. (105 is all the incoming messages as of now since this started happening). I'm completely stuck. Any suggestions?