local host is internal to your machine. That Thunderbird can not connect to it says that their is a third party issue at work.

Some VPN's and proxies use the localhost, as does the local software proton mail installs. We have also seen folk with locally installed software development environments and web server software have issues when establishing a new oauth key as they also often use the localhost adaptor. So I guess you need to look at what software you installed recently and who provides the email on the localhost.

I cleared the cache and the history, and that worked on all but one account, which I deleted and am now trying to add it back. Which is now getting the same error message. I am going to try and install a different account now and see what happens.

I haven't installed anything new.

Thanks for taking the time to answer.

You mention 'yahoo' and 'oauth' and 'localhost'. Yahoo servers would use localhost if it wanted to verify you really were you in order to allow Thunderbird access to server account and set an outh token in Thunderbird. You would not be able to download nor send if yahoo will not play ball until the verifying process has occurred.

Gmail using Oauth would also use the localhost for same purpose and I suspect outlook.com is the same as well.

Before you do anything you need to make sure Thunderbird is not being blocked by your Firewall. It might be the default on computer or controlled by an Anti-virus you installed. Don't try switching off Firewall as that is not exactly proving it resolves the issue besides switching off the Firewall controlled by Anti-Virus does not mean the default one is disabled.

What Anti-Virus are you using?

Exit Thunderbird Access Firewall Thunderbird program must be set up as an allowed program. Confirm this has been done otherwise you may be trying to fix forever.

I do not know what AV program you use - I use Norton and it controlls the Firewall. I added an image showing my settings to allow Thunderbird program.

Please Check you have no VPN enabled. On one occasion I discovered Norton had updated and auto switched on a VPN - I had no idea it done it until I checked - caused no end of problems. Switch off any VPN.

I also have a website which I update and use Apache Xampp - which uses 'localhost' - so if you have anything like that then switch it off until the oauth connection has been established.

Make sure the following is set up in Thunderbird Settings > Privacy & Security Web Content select 'Accept cookies from sites' Accept third party cookies 'Always'

I fixed the problem. I don't know why. Email Gremlins?

The account I deleted and added again then worked.

I use Emsisoft. I had turned it off to see if that was the problem.

I did change the cookie settings. I had set it for places I visited and changed it to always. May I ask why it is better to use the always setting and not the one only just visited?

Thanks so much Leslie