The app could not be started. Close this dialog to ignore the problem or click restart to try again.

Symptom

When trying to launch the Office 365 add-in in Outlook for Windows, you get the error message below:

Solution

This error message can have multiple reasons, please make sure to check each of them. 

Reason #1 - Internet Explorer has been removed from Windows

In case you have uninstalled Internet Explorer 11 from your Windows computer (Windows 7, Windows 8, Windows 10), the add-in will not work in Outlook. Outlook uses Internet Explorer 11 to display the UI, so it's a hard requirement for any kind of add-in to work. To re-install Internet Explorer, please follow the guide on the bottom of this help page: How to use Internet Explorer

Reason #2 - Protected Mode is disabled in Internet Explorer for the Internet Zone

Add-ins can only work with "Protected Mode" enabled in Internet Explorer. If you receive the above error, please check if this setting is enabled. 

1. Launch Internet Explorer 11
2. Go to "Tools" → "Internet Options"
   
3. Under tab "Security", make sure, "Protected Mode" is enabled for the "Internet Zone"
   
4. After changing the setting, a computer restart might be required, if it's still not working out of the box.