Recently Thunderbird asked me to login again for Gmail. It presents me a form in which I have to insert the password. It is the same form as when I login to my Google account via my browser. Only the NEXT button does not work. Do you have an idea how to solve this?

My Win7 had a problem so I had to go back to Win10. And of course the programmers at Microsoft know what is good for us.
I am sure your solution is going to work fine. Of course logging in to the Google account on Win10 takes some time.
Thank you very much.

@T, I'm thinking about a PSA about DNS (that's public service announcement and Domain Name System) since I think I see ISPs (Internet Service Provider) slowing down their DNS or outright hijacking lookups.

You may want to google DNSBENCH and consider changing.