Symptoms

After upgrade to 4.6.9 we could not get the Designer and Runtime sites to work.
If we enter K2 Designer site or any Runtime form, the interface loads but no data is returned. It just keeps spinning indefinitely (infinite spinner).
 

Diagnoses

This seems to be an IIS related issue, after inspecting the Return Body of the AJAXCall we saw that not all data is being returned.

 

Resolution

It turns out there was an ISAPI filter installed in IIS on the K2 site that intercepted the response body for inspection and monitoring but never forwarded this along to the web browser.

So it seemed like things were happening in K2 (for example on one of the Runtime sites a new SQL entry gets created on first load of the page, we could verify this happening) but on the page no data seemed to be coming back.

When we removed this ISAPI filter, the sites started behaving normally again.

The tool is called APM EUEM from the company MCG (http://www.mcg.dk/solutions.html). Please note this tool may be incompatible with 4.6.9 or when upgrading SmartForms, this tool has to be reinstalled/reconfigured as well.