Symptoms

After adding the K2 Worklist app part to a SharePoint site collection, customer gets the following error in Internet Explorer in the app part pane:
"This content cannot be displayed in a frame"

Worklist displays correctly in Chrome.
 

Diagnoses

This is a known Microsoft issue, "Error when viewing apps that utilize web-parts: This content cannot be displayed in a frame", described in the following forum post:

Error when viewing apps that utilize web-parts: This content cannot be displayed in a frame…
 

Resolution

The workaround is as follows:

1) Add “https://login.microsoftonline.com” to the Trusted Sites list in Internet Explorer. Also check that the list of sites below is already in the Trusted Sites list, otherwise add those sites to the Trusted Sites list as well:

• https://<sitename>.sharepoint.com
• https://<sitename>-my.sharepoint.com
• https://<sitename>-admin.sharepoint.com

(where <sitename> is the name of the user’s SharePoint online site. Making this configuration changes will prevent this error from occurring for that user)

2) Clear the IE cache

3) Restart IE