Google OAuth out-of-band (oob) flow is now deprecated (Feb 28, 2022).
jnbrains, FYI, the developers are aware of this issue and are working on a fix. See a recent post by David Fletcher on the Claws Mail users list for a workaround via compiling from source. Otherwise, putting the project in 'Testing' status in the Google Cloud Platform settings still seems to work, although you need to get a new authorization code every 7 days.
I did a troubleshooting session with Google support,
and realized the same temporary workaround. Not very
convenient, yet still accessible for a week at a time.
I will check and test a build from source as soon as I have a bit of time.
In what way is this resolved? Oauth2 with google still doesn't work properly!
You can keep it closed if you like, I've already switched to another email client.