One common cause is anti-virus software. Are you running some? If so, create an exception for Thunderbird.

To troubleshoot, start Windows in safe mode and Thunderbird in troubleshooting mode, and test.

Thanks Rick, I tried exception for Norton AV, didn't solve the problem. I have two AOL emails (@verizon.net) one works fine, the other tells me the number of messages but then shows the pop-up. I also tried Troubleshooting Mode but it didn't change the behavior. Any other ideas? Thanks

If it is Norton, uninstall it and see if the issue goes away. Unfortunately Norton tells lots of lies about what is is doing and is does not disable itself when you ask it to, it only says it does and I do not have the patience to fiddle with the alarmist rubbish to figure out just what part of it is crippling the system more than normal. I would not be so direct, but for it's apparent market share Norton is in dispatches here far to frequently.

Your other choice is to restart your device in Safe mode with Networking (this requires a wired internet connection as Wifi is not available in Safe mode) I offer it second as an option as few these days are using a wire to connect to the internet.

The forum software reports you as using windows, so instructions for Windows in safe mode can be found here. https://support.microsoft.com/en-au/windows/windows-startup-settings-1af6ec8c-4d4a-4b23-adb7-e76eef0b847f