I tried the new Outlook preview version because it has full support for Gmail, including Contacts and the Calendar. However, I experienced two very annoying issues. I spent much time searching for solutions, so I am sharing those here to save you time and effort.

Unable to add another email account

Missing button

I learned the hard way that if the first account you set up is not an Outlook or Exchange email account, you cannot add other email accounts. In my case, I set up my Gmail account first, and the Add Account button was not showing.

Solution

Remove the offending email account and add your Outlook or Exchange email account. Then you can add other supported email accounts (currently Gmail and Yahoo Mail). In order to do so, I had to reset Outlook Preview by running the following in a Command Prompt window:

olk.exe --clearLocalState

Unable to remove a defunct email account with a Single Sign-On screen

I could not progress past a Single Sign-On (SSO) account login screen of an email account I could no longer access. Even though I would close the login screen, it would reappear continuously, and I could not use Outlook Preview.

Solution

As per above, the solution was to reset Outlook Preview by running the following in a Command Prompt window:

olk.exe --clearLocalState

Notes

Using the above command will reset only Outlook Preview. It will not affect the Office 365 version of Outlook.