Symptoms
Worklist control empty, won't load data (infinite spinner).
Diagnoses
Browser console revealed an error
"Uncaught TypeError: Cannot read property 'split' of null"
regarding a line in the Web Control Pack JavaScript bundle
'populateArray:function(t){this.GlobalArrayOfWorklistItems=[]var aq=JSON.parse(this.FilterObject),rq=this.options.hostURL,gt=rq.split("/"),uq=gt[0]+"//"+gt[2]+"/K2Services/rest.svc/Worklist/Items"'
which likely crafts a call to the REST service using the Web Service URL "this.options.hostURL" which was null at runtime.
Resolution
The intended Web Service URL environment field was not set as the default.