Symptoms
Using this documentation: http://help.k2.com/onlinehelp/k2appit/UserGuide/current/default.htm_HowTo_CreateALoopThroughAListOfItems.htm?TocPath=How%20To|_____9
Workflow not updating the correct list items.
Additional errors thrown:
"Unable to cast object of type 'System.Object[]' to type 'System.IConvertible'."
"Object of type 'System.Object[]' cannot be converted to type 'System.DateTime.'"
Diagnoses
One of the generated 'For Each' event's line rules (with conditions) got changed. Re-adding the 'For Each' event does not update this when there are existing outcomes.
The SharePoint reference's Date and Time field is returned as an Object[] and cannot simply be manipulated like a date.
Resolution
Manually added the line rule that is usually generated by the event.
The Date and Time issue was fixed by using the 'First Item' inline function to return the first item of the Object[], which was the date string.