Symptoms

Unable to check in form if Activity level Data Fields are used in the form rules. Disabling Analyzer by means of configuring the following key in web.config:

<add key="Forms.Designer.AnalyzeOnCheckin.DisableAnalyzer" value="true" />

allows to check in the form successfully and use it without any issues.
 

Diagnoses

This is known issue in K2 4.7 (internal ID 712144 "Analyser or ALM is not checking Activity scope datafields") to workaround this you can move DataFields to workflow level/scope.
 

Resolution

Move DataFields to workflow level/scope.