Symptoms

K2 worklist won't render in SharePoint 365 for some users
This Content cannot be displayed in a frame
 

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:
https://social.msdn.microsoft.com/Forums/office/en-US/fa6abb31-7251-4744-ab14-634cde38a42d/error-when-viewing-apps-that-utilize-webparts-this-content-cannot-be-displayed-in-a-frame?forum=appsforsharepoint

 

Resolution

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://.sharepoint.com
• https://-my.sharepoint.com"
• https://-admin.sharepoint.com
(where 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 (Tools, Internet Options, on the General tab click “Delete” under Browser history, make sure to check at least “Temporary Internet files and website files” and “cookies and website data”, delete)
3) Close and reopen IE.