question 2 - no

question 1 - is the precise wording of your error message "There was an error downloading the following message: ... This message may contain a virus or there is not enough disk space. Skip this message?"

No, it said nothing about viruses or not enough disk space. The thread title is the exact wording, and it happens every time I open a folder. I then have to close thunderbird, open it back up again, and then it has to redownload the contents of all my folders. Which sucks up a ton of bandwidth, and makes it impossible to do anything on my computer while it catches up again. This is at least a daily occurrence.

Are you using an anti-virus package? If so, which one?

McAfee home

"Unable to open the summary file for <folder name>. Perhaps there was an error on disk, or the full path is too long." Happened again on all my IMAP folders. Starting over again, redownloading everything again.

Windows has an absolute limit of 256 characters in a path.

What is a path. it is the full location of the file.

So in the case of a Thunderbird profile it is normally

C:\users\[username]\AppData\Roaming\Thunderbird\Profiles\randoms8.default

So tha path to get to your mail is already using 20 or 30 of those characters. Where this becomes a problem is the nested folders in Thunderbird also add to this length.

So a folder names "Aunt Lucy" with a sub folder "anniversary" with a sub folder "all the silly photos of the cousins we were never meant to see" would start to get very close to that 256 character limit.

I think you have already managed to get there.

Have a look at your folder names... to the nests add up to a hundred or so characters?

What is the folder name that generates the error.

Second possibility is McAfee are up to their old tricks again. (it has been years since they had it right.)

Disable email scanning in McAfee as a starting point.

Or, something is deleting all the .msf files. Which would explain the re-downloading.