Otherwise functional gmail account cannot send messages; instead gives login prompt

Dear all,

I have an otherwise correctly working gmail account on Thunderbird 60.9.0 that cannot send messages. I recieve all incoming messages, and drafts and sent messages get synchronized with gmail just fine. But whenever I try to send a message, I recieve a popup prompting me to login to my google account (on the google website; not within TB itself). When I do so, I get a button asking me to grant Thunderbird permission to control my gmail account, and when I click that, the window closes. Thunderbird then does not send the message and instead gives me the error popup "Sending of the message failed!". I have installed this account by following all Thunderbird defaults. My gmail account has IMAP and less secure apps enabled and no two-factor authentication. Any help would be greatly appreciated!