If anyone has an Anti-Virus and they allow the Anti-Virus to intercept all mail to check it is ok, then you will find Thunderbird cannot do anything with the file until the Anti-virus stops intervening.

It's a real pain in the proverbial.

It's possible it is scanning any file that gets opened and in Thunderbird, every time an email is received or sent or saved or moved or deleted etc, a file is opened and updated accordingly. Hence the 'real time protection' is getting very much involved on downloaded files and when files are opened and updated.

Scanning emails is not as helpful as you may imagine because Thunderbird by default does not allow remote content unless you choose to allow it. Thunderbird will not auto open any attachment. You should never open anything or click on anything just because you are curious what it is. Only open attachments which you know who sent it and that it is legit. Emails are not really the problem, but allowing remote content and opening attachments can be. Neither of these occur by default. Only by an action by you.

If an Anti-Virus ever wants to 'fix' anything to do with a thunderbird file then do not allow the fix because it is likely the Anti-Virus will mess up your emails. If you use AV then set up instruction to 'ask' you what to do, so you can stop the fix and sort out the problem yourself. Anti-Virus products do not understand that all downloaded emails are stores in a text file, so one file has many emails. If it fixes a file then you may lose emails.

As you say "If I disable the realtime protection in Windows Defender I have no problems.", so you know what is causing the issue and I can assure you this is a long standing problem when it comes to Anti-Virus products.

Microsft says "While real-time protection is off, files you open or download won't be scanned for threats."

Suggest you first try this - make the Thunderbird profile folders are exempt from scanning. See if it makes some improvement. Info here:

• https://support.microsoft.com/en-us/windows/add-an-exclusion-to-windows-security-811816c0-4dfd-af4a-47e4-c301afe13b26

Thankyou for your solution, but it did no help. A couple of days I used the realtime protection of Malwarebytes. After enabling again Windows Defender the problem with Thunderbird has been solved. I do not know how that is possible.

Thankyou for your solution, but it did no help. A couple of days I used the realtime protection of Malwarebytes. After enabling again Windows Defender the problem with Thunderbird has been solved. I do not know how that is possible.

My guess is defender got a virus update that stopped it being a fool and basically closing the system up for business. These antivirus products get frequent updates, in some cases hourly and the updates can turn them feral, or fix that in the blink of an eye and the vendor never admits there was a problem in the first place. So like you folk think it "just happens"

This morning I analyzed the files in the profiles-folder. I saw that the file addonStartup.json.lz4 has been changed before the problems arose. I have profiles-folders in Documents and Settings as well in Users. Now I have all profiles-folders of Thunderbird exempt from scanning by Windows Defender.