Symptoms

After installing the K2 for SharePoint app, the registration wizard fails with the error "VALIDATIONAccess denied. You do not have permission to perform this action or access this resource.Failed to initialize the Context:......" The user running the app registration is a site collection administrator.
 

Diagnoses

This can happen when a new version of the app has been deployed to the App catalog, but the existing app on the site has not been updated to the new version.
 

Resolution

Update the app by mousing over the app, clicking on the ellipses and clicking "About". This will take you to a page showing that a new version of the App is available. Click "Get it", then "Trust it", which will update the app.

You can then run the registration wizard successfully.