Remove the entries for the account from Saved Passwords in Settings/Privacy & Security, accept cookies in TB Settings/Privacy & Security, restart TB, enter the new password in the OAuth window.

I removed the entries for that account. When I restart TB, a window pops up with title "Enter credentials for <my gmail account> on imap.gmail.com". The only place to enter text is labelled "Email or phone". I can enter my email there, and click Next, and then I get the message I mentioned.

Just tried again, and got "Network protocol error" instead.

Tried again, and go the original message: "400. That’s an error. The server cannot process the request because it is malformed. It should not be retried. That’s all we know."

I've made some progress now. I didn't have cookies enabled for this account. Enabling them let me get to the next step, where it wanted to use the Passkey that I just set up. That's connected to the fingerprint sensor on my laptop. However, I couldn't get it to prompt me to use the sensor, so I had to select "use another method" (for which I chose confirmation via my phone).

This now works. I've logged out and back in, and it didn't prompt me again.

So the main issue was that I hadn't been allowing cookies.