Getting 2400 error in outlook application

I hear your pain as it did also start for me recently. Have had two @outlook.com in my profile forever and now I just get errors.

Unless we take this to social media, Microsoft is not going to take us seriously. I am a small partner and not even because of that, I get any help. I was told to contact UNIFIED SUPPORT and PAY! .....

I can open email@hotmail, @gmail, @yahoo, but not @outlook using Outlook application on Windows 10. I was OK until 3 weeks ago, the Outlook on Windows started giving this error. At the same time, I cal open @outlook email online or Windows' Mail application. It is the last office update that done this?

My PC, Dell Inspiron 3670, 4yrs old, running Windows Home and MS Family 365, is experiencing the same 2400 error. My wife runs Windows Pro on a 2yr old Dell XPS with the shared MS 365. She has no issues with her Outlook. I went thru an extensive 1 1/2 hr session with a MS tech with no joy at sign off.

I just want to add that Outlook runs without error codes on my android phone and tablet. Just saying.

I am one of those who have ordered a new PC using this Outlook error 2400 as an excuse. Hoping I haven't spent $1.1k on a new machine that presents me with an Outlook 2400 error code.

Good luck all...

Yesterday morning, a number of users all reported that they were getting pop-ups in Outlook to sign in. These pop-ups are the Microsoft/O365 type pop-ups, and not the Exchange login ones. When users try to sign in with their O365 credentials, it gives them an error message CAA2000B, and when they click continue, it gives them Error 2400. This popup comes right back if they dismiss it. The issue persists as of this morning, and no new Office or Windows updates are available today

In order to troubleshoot this, we have verified correct O365 licensing, confirmed users can login on office.com and also our webmail client indicating that both their Exchange and O365 logins are working.

On the client computer side, we have tried a significant number of issues:

• Verified behavior on multiple internal and external networks (guest wifi, phone hotspots, etc.) and LAN connection
• For users who have profiles that were working, ensured Outlook was authenticating the correct mail server in Connection Status
• Removed O365 account from Word, launched Outlook to do profile setup.
• Opened word back up, re-added the O365 account and ran Outlook for profile setup again
• Deleted Outlook folder in Local App Data
• Made a new Outlook profile (sign-in fails in the same form when making the new profile)
• Selected Exchange and Exchange 2013 or earlier during profile creation
• Ran all Windows and Office updates
• Ran Office online and full repair
• Disabled AV/Firewall/Threatlocker and ran Outlook as Admin for setup
• Did all the same + ran from a local admin account
• Set DUO (our 2FA) to bypass
• Set a registry property in HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover : Dword ExcludeExplicitO365Endpoint value 1
• Used about a dozen options in the Support and Recovery Assistant in the Office and Outlook categories
• Reinstalled office
• Tested this with a completely new PC with a fresh Windows install. Did not add any company software or domain connections before doing this to ensure no GPO/DNS/etc. would get in the way.
• I had one user upgrading PC's yesterday anyway, so I took their old computer and uninstalled office, completely deleted the registry folder HKCU\SOFTWARE\Microsoft\Office\16.0, cleared all credentials, restarted the PC, reinstalled Office, and set up the accounts again. Set-up failed with the same errors.
• I have one user account I have tried all of these fixes on, all return CAA2000B then 2400 or just 2400.

Anyone have any ideas?

Try to delete email account in control panel. Then add it again.
I set a PIN of this account from other computer, thus might get an authorization problem.