Symptoms

Unable to configure K2 for SP app once it was added to SP site
 

Diagnoses

We're unable to configure K2 for SP app once it was added to SP site. Once I click on the app icon to start the configuration it keeps prompting me for credentials (I've made sure they are correct!). I've tried providing credentials for k2 service account, SP system account as well as SP site collection admin.
If I exit the prompt dialog, there is not error, so it's hard to figure out why is it not accepting anything.
 

Resolution

Disabling loopback check and adding the appropriate sites to the trusted zone resolved the issue.

1. Is "DisableloopbackCheck" or its equivalent implemented on the SharePoint server?
http://support.microsoft.com/en-us/kb/896861

2. Please also added the correspond sites (sharepoint url , appdomain url , and K2 url) to the trusted zones in your browser setting, ex:

*.domain.com
*.appdomain.com