Symptoms

When opening K2 Workspace, it throws the following error:

Server Error in '/Workspace' Application.

This operation requires IIS integrated pipeline mode.
 

Diagnoses

We first found that the K2 server was accessed through the internet, so there are some proxy servers involved. We tried switching on "Compatibiliy Mode" for the K2 Workspace site, even put it in the "Trusted Sites", it didn't resolve the issue. We also tried switching off or bypassing the proxy servers but that just prevented the site from opening at all.
 

Resolution

We finally figured out that if we enable Enterprise Mode under Emulation settings in F12 Dev Tools in IE, it will open K2 Workspace. It turns out there is a Group Policy setting that prevents sites from being opened in Enterprise mode, so this site had to be added to a XML file, like so:

andltdomain docMode="9"andgtk2.domain.comandltpath docMode="9"andgt/workspaceandlt/pathandgtandlt/domainandgt
andltdomain docMode="9"andgtk2dev.domain.comandltpath docMode="9"andgt/workspaceandlt/pathandgtandlt/domainandgt

After applying this change, the K2 Workspace site automatically opened in Enterprise mode via Group Policy which resolved the issue.