Symptoms

When opening any SmartForm for the first time, a login page is displayed allowing users to choose from security token services, specifically Windows STS or ADFS. However K2 should automatically login with ADFS.
 

Diagnoses

Both "Windows STS" and "ADFS" have been set to "true" for "UseForLogin"
 

Resolution

Using the management SmartForms located in K2 Designer to update the Claim Issuer values. This form is found at System -> Management -> Security -> Forms -> Manage Issuers

We set the "UseForLogin" property to false for the unwanted STS.

Regarding the permission issue (need Contribute permissions on the site) we added the user account directly to the root site permissions instead of in a group. We also did "Synchronize Groups" inside the K2 app.