Symptoms

All AJAX communication done by the K2 for SharePoint app is not working any more.
Calling the K2 for SP App settings page or any K2 library application page from the ribbon always leads to the following error:

Unexpected response from server. The status code of response is '0'. The status text of response is ".
 

Diagnoses

We were going to need a lot of logs to analyse this issue, including the following:
- F12 Dev Tools in Internet Explorer (Network tab and Console tab, anything with an error)
- Fiddler trace
- Could be certificate error (Fiddler would expose this)
- Could be proxy server related
- Run AppDeployment.exe on SharePoint server to check if Pre- and Post Deployment analysis succeeds
- It might be a firewall issue, port 6332 needs to be open to deploy K2 app. See this KB: http://help.k2.com/kb001318
- Enable full Host Server logging
 

Resolution

We found that there had been CORS errors for each AJAX call the app executed. As the SharePoint farm is operated by another company, we sent all the hints to them, and they finally managed to resolve the issue. It turned out that the authentication settings on the app domain were messed up (possibly because anonymous authentication was disabled), which led to the CORS errors.