Microsoft told me this is a bug in the current O365 release and should be fixed in the next patch.

As a workaround, they gave me a registry key to deploy and it has worked for all users that have received it. I did not have to make the users enable the Microsoft exchange add-in as it was already enabled in our environment by default.

HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\Options\CalendarAdd: ShowLegacyRoomFinder with dword value 1

Here is the PowerShell script i used for deployment in case anyone needs it:

Set variables to indicate value and key to set

$RegistryPath = 'HKCU:\Software\Microsoft\Office\16.0\Outlook\Options\Calendar'

$Name = 'ShowLegacyRoomFinder'

$Value = '1'

Create the key if it does not exist

If (-NOT (Test-Path $RegistryPath)) {

New-Item -Path $RegistryPath -Force | Out-Null

}

Now set the value

New-ItemProperty -Path $RegistryPath -Name $Name -Value $Value -PropertyType DWORD -Force

You can deployed it via GPO as well

Here is the article with details.
https://support.microsoft.com/en-us/office/how-to-revert-the-outlook-desktop-webview-based-room-finder-to-the-legacy-room-finder-e872b6f2-0d36-41ff-861d-adaab2da9c28#:~:text=How%20to%20revert%20the%20Outlook%20Desktop%20WebView%20based,Close%20Registry%20Editor%20and%20restart%20Outlook.%20See%20More.

Hello, we have some users who are having this issue. Was there a proper resolution to this yet?

Thank you

We are having the same issue, although with some additions. We get users having a browser popup with error 500 when creating new meetings and users having error 400 popup in a browser when they open outlook or try to edit some messages.

I was able to take care of some of the ones related to the meetings with the following regedit in a gpo

Key Path: HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options\Calendar
Key: ShowLegacyRoomFinder

Type: REG_DWORD

Value: 1

I have noticed that the other errors are occurring on this web page:

https://login.microsoftonline.com/common/oauth2/authorize?client_id=00000002-0000-0ff1-ce00-000000000000&redirect_uri=https%3a%2f%2foutlook.office365.com%2fowa%2f&resource=00000002-0000-0ff1-ce00-000000000000&response_mode=form_post&response_type=code+id_token&scope=openid&msafed=1&msaredir=1&client-request-id=f322cfb1-88cf-79b5-3f43-09e2b277fd8b&protectedtoken=true&claims=%7b%22id_token%22%3a%7b%22xms_cc%22%3a%7b%22values%22%3a%5b%22CP1%22%5d%7d%7d%7d&nonce=638059384817229913.d8a9da8a-7062-4905-ab94-7f8c74284040&state=DctBEoIwDEbhVu7hDWpDG0iycDjLr6iwgHYGZri-XXxv97xzrmtujacWJ2NWGiwray8pmfX5MStshiIIjSmw0RDwMg7y1bdwUiYm3957LBfiVHEuz1jquZb9iBt2_D6o9fgD&sso_reload=true

I can access this webpage fine on my computer, and mine does not get the popup when I open outlook or when I mess with messages like some users do. Like others have said here I have noticed this occurring ever since recent updates.

I also noticed the error occurs if I go to File and then click on the button that says "Manage Add-ins" : "Manage and Acquire Web Add-ins for Outlook".

I have already tried repairing, reinstalling, running outlook with no add-ins enabled, creating a new outlook profile,, flushing dns, clearing browser cache, verifying network settings are correct for the users having the issues.

Email and calendar works, so I am not sure why these browser popups are necessary, especially when opening outlook.

Hope this info helps to find a resolution

Sorry - Only Troubleshooting steps I received from my ticket with Microsoft have been to Revert office version and change default browser. We have also started getting users with Calendar issues beyond this browser windows opening that has not been solved - inability to type consistently or copy and paste into the meeting invite windows. These users are users with the browser window opening as well.

Unfortunately, that did not work for me.