is the account IMAP or POP?

imap-mail.outlook.com

I have the same problem. I'm using IMAP, and this message pops up every time I start Thunderbird. Suspecting a corrupted file, as some forum posts suggest, I completely deleted the account in Thunderbird, deleted the data and created a new account. The messages all downloaded anew from the server, but next time I opened Thunderbird ... the same problem happens. So I think it's TB, not the file. Using TB 60.4.0. Before I replaced the account I downgraded to TB 52, thinking it might help, but it made no difference, so I re-upgraded.

The message generally has one of four causes.

1. An add-on. Restart with add-ons disabled from the help menu and observe.

2. Anti virus locking files to scan them while in use (create an exemption in the security product to stop the contention.

3. Data is stored on a drive that is constantly syncing. More and more folk think they can share their profile using verious web drives. They really can't but they do it anyway.

4. The global indexer is having issues. Turn it off in options > advanced > General. It may be that the global index itself is corrupt but determine if it is the issue before you more on.

My AV (ESET) is set not to scan emails. The data file is TB's default, in AppData, which doesn't sync with anything. The global indexer is turned off. That leaves an Add-on. The problem only occurs on my laptop, not my desktop. I thought the Add-on profile for both were identical, but I see I have Awesome Auto Archive on the laptop and not on the desktop, so I have removed it and we'll see what happens.