Symptoms

Form Viewer and Worklist Web Part error: 'The website cannot display the page -- HTTP 500'.
 

Diagnoses

Network trace revealed that the 500 error occurred on the URL with the following structure:
https:///AppDomain].sharepoint.com/sites/Appcatalog/K2AppitforSharePoint/Pages/sSFRuntimeURL]/Form/K2 Worklist/?_theme=SharePoint 2013andupn=nUPN]andtrust=urn:federation:microsoftonlineand_spmod=1

The SmartForms Runtime URL in '/Lists/K2 Settings/' was malformed, containing an tag with a link to the wrong instance. This was getting passed literally, tags and all, thus returning the 500 error.
 

Resolution

Couldn't activate the root site collection it returned 'Warning: Different K2 Server Found". Backed up the the 'K2 Settings' list on the root by going to 'List Settings' -> 'Save list as template' -> 'Include Content', then deleted the list. Re-ran activation on the root site and it completed, creating a new 'K2 Settings' list with a SmartForms Runtime URL correctly pointing to the Prod server.