This article was created in response to a support issue logged with K2. The content may include typographical errors and may be revised at any time without notice This article is not considered official documentation for K2 software and is provided "as is" with no warranties.
When navigating to K2 Designer, a Server Error message for the targetFramework node appears.
K2 Smartforms Application Pool was using the Microsoft .NET Framework Version 2.50727.8762.
Seeing from the screenshot, it was somehow set to Version 2.50727.8762.
Updating the K2 Smartforms application pool to use the .Net Framework 4.0.30319 resolved the issue.