We are running SharePoint 2013 in 2010 compatibility mode. K2 Smartform viewer webpart is not loading SmartForms and an error appears in K2 4.6.11:
"Your browser's compatibility mode is not supported"
After removing K2 SmartForm from Compatibility Mode as described in this article: https://help.k2.com/kb001704.aspx, the issue persists.
This issue may be caused by SharePoints master page compatibility.
In addition to the change in the website you linked to, please change the SharePoints master page compatibility meta tag to IE=11 instead of IE=8.
Note that this should be done specifically to pages that house K2 Forms as changing the document mode in SP 2010 can make some ribbon controls not function properly.
Additionally, you may be able to change the document mode via IIS, specifically for the K2 Runtime site, by modifying the 'X-UA-Compatible' HTTP Response Header on the K2 - Runtime site in IIS. The field should have values similar to the following: "IE=11,IE=10,IE=9,IE=8".
Removing the lower versions would force Internet Explorer into a modern Document Mode for Runtime SmartForms, which may resolve this issue.