Issue

After installing the May or June CU for SharePoint 2019, you will receive an x-frame option error when adding the K2 for SharePoint App from the App Catalog.

Resolution

1. Open Internet Explorer in compatibility mode and add the K2 for SharePoint App from the App Catalog
2. In other browsers perform the following steps:
   1. Go to IIS Manager on your WebFrontEnd server(s).
   2. Click on the Site
   3. Click the HTTP Response Headers
      
   4. Add the following header :  Content-Security-Policy : frame-ancestors 'self'
      
   5. Remove any other deny header
      
   6. Recycle the worker process