Have you used IMAP before with this gmail account? If not, then you may need to enable imap in your gmail account settings. Go into this gmail account using a web browser, select the settings icon (cog/gear) in the top right corner and then Forwarding and POP/IMAP from the top menu.

Thank you, KD Johnson, the answer to the question is YES. I should have been clearer in my description. Sorry! I get this message when I switch to POP in Gmail. If I want to do the same in Thunderbird, it's not possible. There seems to be some kind of preselection (see picture). Is it possible to do something about it? /hans peter

The solution in Thunderbird is first to delete the gmail-account and then add it again. In that procedure, you can choose between POP and IMAP. /hans peter