Could you be a litle more specific. Does TB start at all?

Yes, it does. I can send, but not receive. It is just trying to download endlessly...

Actually, first all folders disappeared. But it was downloading, I just could not find anything. I have reinstalled it, then I saw all folders but I could not send. I reinstalled. Again downloading, no folders. Reinstalled - I have all folders but not downloading. I tried to find someything on the internet, but none described the same problem.

Please Start *Windows'* safe mode with networking enabled - http://windows.microsoft.com/en-us/windows-10/change-startup-settings-in-windows-10#v1h=tab01

Still In Windows safe mode, start thunderbird in safe mode - http://support.mozillamessaging.com/en-US/kb/safe-mode

Does problem go away?

Thanx. I am away until next week, I will try it then.

Thanks. Looking foward to your report

Strange solution: I had to change the connection security in the server settings to 'none'. Nothing changed, only difference, I installed W10...... Hmmmm.

It is my opinion that on updating, Thunderbird occasionally loses (or changes) a user setting. It might be server security, authentication, toolbar visibility, html default message composition, etc.

Was it set to "none" previously? Had it strayed to a different setting?

Yes! I have checked it on another computer. It was indeed 'none' previously. I should have checked it earlier, it just did not occur to me that the settings could have changed........ (It was changed to 'starttls'...... ) In spite of already having met this 'settings changed" problem with other programs.... Thanks for the help!