Hmmm. I don’t know exactly what’s going on, but I might be able to enable you to find out more information. I’m guessing your popup is from a Skuid page where you’re attempting to access data from an Outlook model? If so, try opening your browser’s developer tools, go to the Network tab, and then refresh the page. You should be able to see something that looks like this (I’m on Chrome):
This tab will show you some of the requests being made and answered on your Skuid page. If you’re trying from somewhere else you might still be able to check the Network tab. Sometimes, you can check there for messages in red that will give you more information about why something went wrong (in your case something is going wrong with the authentication process). Would you mind posting a screenshot of the network section if you see any items in red? You can also click on an item in the list to get more detail.
Are the two login screens the exact same, and if so are they the standard login page?
One more thing you might check - that the authorize endpoint of your Outlook authentication provider is https://login.microsoftonline.com/common/oauth2/authorize?resource=https%3A%2F%2Foutlook.office.com%2F
