Can't add gmail imap account on Ubuntu 16.04 - the google auth popup's "next" button does nothing when clicked.

Using Thunderbird 45.8.0 on Ubuntu 16.04. Adding existing gmail account: I enter my name, email address and password. Clicking "Continue" autodetects gmail's IMAP servers. Clicking "Done", Thunderbird opens a browser-like popup (see attached image) where I am asked to "Sign in to continue to Mozilla Thunderbird Email" My email address is filled in by default, so I click the button "Next", but absolutely nothing happens - as if the button wasn't clicked at all.

I have internet connection. I have restarted my PC a couple of times. I have IMAP enabled in my gmail account. I have "less secure apps" turned on in my gmail account.