Symptoms
'X' close button not closing subform with IE=Edge document mode.
Diagnoses
Form closes successfully with lower document modes.
Resolution
Force IE10 Document Mode by modifying the 'X-UA-Compatible' HTTP Response Header on the K2 - Runtime site in IIS. The current value was "IE=11,IE=10,IE=9,IE=8". Removing 'IE=11' forces Internet Explorer into IE10 Document Mode for Runtime SmartForms, which should resolve this issue.