Activating the K2 for SharePoint App on a SharePoint site collection results in an error on the activation step that does not return any error message and does not actually get to the point of activating any sites or subsites in the collection.
This only happens on a specific site collection, other site collections activate without issue. A network/Fiddler trace of the error will return a 500 error on an AJAX call and Host Server logs will log the following error message:
An error occurred in the SharePointIntegration Service Instance. System.ArgumentNullException: Value cannot be null.
Parameter name: title at SourceCode.SmartObjects.Services.SharePoint.Integration.Execution.Execute.ConfigureSpoProvider(String title, String siteUrl, String tenantId, String resourceId)
The stack trace here is the key component. Note in the Host Server log error:
"ArgumentNullException: Value cannot be null.
Parameter name: title at SourceCode.SmartObjects.Services.SharePoint.Integration.Execution.Execute.ConfigureSpoProvider(String title, String siteUrl, String tenantId, String resourceId)"
This ConfigureSpoProvider method (which may be named differently if you are running SharePoint On-Prem) calls the SharePoint Group Provider that is stored in K2 Management. An error such as this indicates that something is wrong with this site collection's group provider.
To see if this might be the case, navigate to K2 Management > Users > SharePoint and make sure the Name and Site URL values for the site collection's group provider are valid.
If the Group Provider Name value is blank, browse to the site collection > Site Settings > Look and Feel > Title, Description, and Logo and verify that there is a valid entry for the site collection title.