Emails are temporarily not displayed

Start by blocking any antivirus scanning of on access or change in the Thunderbird profile folder. It has a very long history of causing data loss, file corruption and is generally a poor idea. Personally I dumped paid antivirus for windows defender years ago and have had no virus issues nor related email issues caused by the antivirus product since. I see it as a win win. It is entirely possible that simply displaying the email sees your antivirus blocking the local temp folder while it scans it. Display of an email almost always included at least one temp file. You might also want to consider a manual cleanup of the temp folder as windows makes no attempt to clean this itself by default and it is a primary cause of the old stories that do the rounds of the internet suggesting Windows gradually slows down after it is installed. The temp folder becomes so full of junk Windows noticeably slows down navigating the tens of thousands of orphaned entries and other oddities appear as a result. Microsoft offer instructions on how to automate cleanup and how to initiate it manually here https://www.microsoft.com/en-us/windows/learning-center/how-to-delete-temporary-files