Symptoms

When clicking on “Design a new workflow” option in K2 designer error message “Server Error in ‘/Designer/Workflow’ Application” is being displayed. Error message details:

Server Error in ‘/Designer/Workflow’ Application
The resource cannot be found
Description: HTTP 404
Requested URL: /Designer/Workflow/_trust/Login.aspx
 

Diagnoses

Such error can possibly appear after unsuccessful attempt to configure ADFS authentication. Things to check are Claims and Issuers as well as IIS authentication settings. You may do this via respective forms which you can find via search in K2 designer (preferred approach) or by checking of ClaimRealm and ClaimsIssuer tables in K2 database (in case you are going to change something you have to be sure that you understand what you are doing, and made backup of your K2 database beforehand).
 

Resolution

1) Any erroneous or unnecessary entries have to be removed for Claims and Issuers.
2) Designer site authorization settings in IIS has to be set to use Forms and Anonymous authentication (as well as Runtime and ViewFlow sites)