I also tried connecting my gmail to K-9 and had this error. At first i thought it might be my deactivated OAuth2 but still after activating and trying in a new setup, it still doesn’t work. This is the error screen:
This error means that you cannot sign in temporarily, but it seems to be a problem on the application side, as K9 does not allow you to log in, but Thunderbird does.
If you are using the current version of K9, you can use it by switching to Thunderbird.
However, the current K9 and Thunderbird UI is terrible, so I recommend you to use FairEmail.
Hi folks, we have some more info on this matter at Gmail and Thunderbird for Android | Thunderbird for Android Help which I hope will be helpful. We’re working to get K-9 Mail back on track, but there are some hurdles Google put in our way that will take a while to work out.