Symptoms
After installation of the new environment when trying to open from K2 designer Runtime URL for a test form or when opening Runtime URL for Manage Site Realms form forms look strange and most of the graphical elements are missing.
Diagnoses
Based on symptoms it could be the case that Runtime site in IIS doesn’t have Anonymous authentication enabled. It is necessary for Designer, Runtime and ViewFlow sites in IIS to have both Forms and Anonymous authentication enabled in IIS.
Resolution
Enabling Anonymous authentication for Runtime site and IIS reset resolved the issue.