See if this helps. https://support.mozilla.org/en-US/questions/1271866?page=2#answer-1266662

I have the same issue too. I do not know exactly when it started, but I would say at least 2 years ago.

I am not a Kaspersky user. I tried installing a different version (I had the 32-bit and moved to the 64-bit), changing drive on which Thunderbird is installed, using a tool that checks files are not locked by any other program, made sure that the firewall would not block anything coming from TB and disabling the antivirus software, without success. I can, however, manually download a new version by clicking on the blue button in the notification and overwrite the current version with no issue, so I do that every now and then.

Of course what did not work for me might work for you but I wanted to save you some time in case you were thinking about trying some of these steps. Fingers crossed that eventually a solution is found!

I chose christ1's reply - thanks! Following the suggested topic and having Avast, I added the following exceptions (Menu/Settings/General/Exceptions): https://aus.thunderbird.net/* https://aus5.mozilla.org/* Since Sep. 1 there is no repetition of the problem. I believe it may be valid for other anti-virus software too. Strange is that I have a dozen of machines with Avast and Thunderbird with the same settings, but the problem occurred on only 1 of them. And I still think it is a Thunderbird issue…