SignInResponse error on first login

Problem

The following error is shown whenever a user clicks on a link to a form and navigates to a K2 form for the first time:

Server ErrorID3206: A SignInResponse message may only redirect within the current web application: '/Workspace' is not allowed.

Solution

Check if you have configured the following web applications under your K2 site to use IIS Windows Authentication instead of Forms Authentication:

RuntimeDesignerViewFlow

By default, K2 setups those web applications to use IIS Forms Authentication. But if for some reason you are required to use Windows Authentication instead, you need configure the following:

1. Go to K2 Management > Authentication > Claims > Issuer, uncheck 'Use for Login' for both 'K2 Windows STS' and 'K2 Forms STS'

2. Open IIS Manager on your web server, select Runtime, select Authentication. Disable Anonymous Authentication.

3. Repeat step 2, but for Designer and ViewFlow. You should not need to make the same change for Management and Workspace since they use the same configuration as Runtime, but you can still go ahead and make sure Anonymous Authentication is disabled.

4. Perform IISRESET