It could actually be the antivirus that's interfering with Thunderbird's ability to make changes to the file where the folder contents are stored… Start Thunderbird in <b>Troubleshoot Mode</b>. The following article explains how to do that and why it can be useful to help identify the cause of a problem: https://support.mozilla.org/kb/troubleshoot-mode-thunderbird Start Windows in <b>Safe Mode with Networking</b> mode. You may have to connect your computer to the router with an Ethernet cable, WiFi is disabled in Safe Mode: https://support.microsoft.com/help/12376/windows-10-start-your-pc-in-safe-mode Does any of that make the problem go away? If Thunderbird shows the error message when running normally, but not in Safe Mode, that would be a clear indication that the antivirus or some other piece of software you have installed is interfering with Thunderbird.