Symptoms

SharePoint List K2 App integration error
 

Diagnoses

The issue here occurs when you generate a new application from a sharepoint list, creating smart objects and forms you then exit the application. If you then click on the K2 ribbon then you will see that all the applications created previously go missing from here. However if you check the K2 Designer you can see all of the created forms.
 

Resolution

This issue occurs due to the new Sharepoint styling. It will be best to revert back to classic mode to create the K2 applications and then after you can revert back to the new styling. If applications have already been created, revert back to the old styling and upon creating the new application choose to overwrite the existing smart objects to bring back the already generated artefacts here.

This has been linked to an existing known issue and will be within future releases of K2.