Symptoms

Smartforms does not load when Smartforms URL is embedded in a Sharepoint Viewer Webpart or K2 Smartforms Viewer Webpart (spins indefinitely) loading this Smartforms outside the context of Sharepoint is working successfully. This is Smartforms 4.6.9. 

Diagnoses

Please see the KB article below:
http://help.k2.com/kb001696

Starting with K2 Smartforms 4.6.9, Internet Explorer 8 Document Mode is no longer supported as such it maybe that the browser is running in IE8 Document Mode due to a Compatibility View Settings or the Sharepoint 2010 page/master page document mode is set to IE=8 by default. One can use the browser's Developer tool to see which mode it is running. 

Resolution 

Below are the possible workaround if you wish to load Smartforms within Sharepoint (using the Sharepoint Viewer Webpart or K2 Smartforms Viewer Webpart) as per above KB:

• Use a later version of Internet Explorer.
• Use a different browser.
• Prevent K2 smartforms from running in Compatibility mode or Enterprise mode in Internet Explorer by using Edge Mode – see KB001704 on how to configure Edge Mode.
• Get the available cold fix from Regional support (this coldfix will only change the behavior instead of the infinite spinning icon, an "Your Browser's compatibility mode is not supported" error will be displayed with a direct link to the Smartforms URL).

As a workaround, it was decided to use the SharePoint 2010 Worklist web part.