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.

THANK YOU rostosha

4 hours on the phone with apple and they could not fix this issue, Google doesn't even have a team for this. Terrible support

This will all depend on browser you have set as default on your iMac, Mac, or MacBook device. If its Safari, from the top task menu, make sure you see Safari listed, click on the Safari option, then click on the Privacy tab if you are on the latest version of MacOs Sonoma 14.2.1.

Then uncheck two boxes:

Website Tracking: Prevent cross-site tracking

Hide IP address: Hide IP address from trackers.

Go to the Contacts app and enter your credentials for Google. When you see all your contacts list, you can go back and enable the settings within Safari. I have not had any issues since two days ago.

This appears to be working, or at the very least my iMailbox is now downloading all my Gmail and sub folders. Hasn't finished yet but upon completion I will attempt to switch back to Safari as my default browser and see if it holds up. Thank you!!!

This issue only showed up after Google started pushing PassKeys and changed their security settings page. Something on their end messed this login process up.

Hopefully Apple and Google can figure it out and fix this issue as I think it will become a much bigger problem over time. On Apple's end - they should adjust any privacy settings blocking the ability to add the account in private browsing mode. On Google's end they should coordinate with Apple to make sure security changes work with various OS'.

I tried all the different privacy settings and content blocker features and was unable to get it working on Safari.

I used Chrome instead - set it to default, signed in and added the account, then switched back to Safari as the default -- as rostosha mentioned.

Google is doing everything it can to FORCE people to use Chrome. In my situation, I could not access my Gmail account with Apple Mail. I was able to log into my account on the Gmail website. I think I tried just about everything suggested here. The "simple" fix was to make Chrome my default browser. Then all of the Google authentication gobley-goop went through just fine. After achieving this goal, I eagerly switched back to Firefox as my default browser.

We all know, Google would NEVER cause these problems intentionally.

I was having this issue after I changed my gmail password. I kept getting asked for my google login for internet accounts. I was able to resolve it by going into the privacy tab in safari settings and click on manage website data and removed all.

In the end I really do think that this is a cache and issue so changing you default browser would work if it’s never been associated with your Gmail account. Changing browsers didn’t work for me and it seems like there are a few work arounds out there so that’s great!

I am trying to correct error 400 that prevents me from using my Google mail on my iMac. When I go to Safari Desktop & Dock and try to change the default browser I only see Safari. How can I add Chrome or another browser?

I changed the password on my gmail account for security reasons. When I tried to set the password on my Mac mail, I got this same 400 error message. I then set my default bowser to Chrome as suggested and repeated the same process to set the password on Mac mail. The process opens up Safari, even though I set the default browser to Chrome. So it didn't work, and I get the same 400 error message

If you go into Safari --> Settings --> Privacy --> Advanced and uncheck "Privacy: Use advanced tracking and fingerprinting protection" you will be able to sign into your e-mail. You can then enable this feature again once you've successfully logged into your e-mail. There is no need to install chrome or change your default browser.

When your logging into gmail with Safari by default it launches a private browser to finish the transaction and having this feature enabled blocks whatever google is doing on the backend to validate your session.

@rostosha - This worked perfect for me! I had the issue for weeks. I did your steps to default to Chrome, then added my Gmail account and worked. Then just put the default back to Safari after it worked.

My goodness, what trickery! This worked perfectly. Thanks.

Thank you! This was driving me nuts. After I changed all my email passwords, one of my accounts gave me this error (just one). Changing to Chrome for default browser was the only thing that worked.

Don't forget to change it back to Firefox or Safari when you get things working.

Thanks but that did not help at all.