There's an old thread here that might be relevant: https://support.mozilla.org/en-US/questions/993843 If you read through it you'll see that the cause was tracked down to a security add-on in Firefox (not Thunderbird) that was preventing the Thunderbird 'welcome screen' from being displayed. Another way to explore this would be to look under Preferences > General. At the top there should be an option to show the welcome screen when Thunderbird starts. You could try un-ticking that option, closing and restarting Thunderbird and seeing if you still have problems.

That certainly stopped the error message, but the get messages times out connecting to the pop3 server and sent mail is rejected. That suggests that the settings are corrupt. Ive checked the setting, which are correct but I cannot find where to change/check the password to the POP3 server?

Preferences > Security > Passwords > Saved Passwords should show you all the passwords Thunderbird knows.

Its not the passwords, they are correct. But for whatever reason, Thunderbird (on this machine) is not able to access the pop3 mail server. If I delete thunderbird, and reinstall it afresh - will I lose all the mail and contacts?

Your settings, messages, address book etc are stored independently of Thunderbird in its "profile". You can, in principle, remove and re-install the Thunderbird program without affecting the profile. But because the settings and configuration are stored in the profile, re-installing Thunderbird is usually pointless.

Having said that, some users do seem to be able to blow everything away. It's always wise to back up your profile before starting, so you can at least get back to where you are now.

https://support.mozilla.org/en-US/kb/profiles-tb?esab=a&s=profiles&r=1&as=s

So basically I'm unable to resolve the issue and need to install a new mail program? It seems Thunderbird has become inoperable on this machine. Not good.

Could you try running Windows in safe mode with network enabled. Then try T-bird. Just want to check if its related to your firewall or virus protection. Some firewalls assume an update is a completely new program and doesn't allow it thru.

Thanks, sorry for the delay, it raised another issue - Windows 10 in safe mode wont accept my password or pin, which I gather is a common issue. Got to sort that first. BTW, Thunderbird updated all its mail at midnight, but then refuses to connect to the pop3 server to send or receive again now - so it seems its a firewall/software issue somewhere.