Symptoms
After the 4.7 upgrade, I installed K2 App on App Catalog Site and registered it successfully.
When trying to activate App on site collections and gives: The remote server returned an error: (400) Bad Request.
The SharePoint 2013 environment doesn't have the latest SharePoint CU update.
Diagnoses
This issue can occur when SharePoint environment was not updated to the supported SharePoint Cumulative update for Blackpearl version 4.7.
Resolution
The SharePoint environment will need to upgrade to at least SharePoint 2013 CU = KB3127933.