Hi,

Out of curiosity,  does it work if you use the SharePoint Page Viewer webpart? The K2 for SharePoint integration has two out-of-the box webparts (K2 Smartforms Viewer and K2 Worklist). If the environment is already integrated with K2 for SP app, then I would recommend using the K2 Smartforms Viewer webpart.

https://help.k2.com/onlinehelp/k2five/userguide/5.3/default.htm#K2-for-SharePoint/WebPartsAppitSP.htm%3FTocPath%3DAdminister%7CK2%2520for%2520SharePoint%7C_____2

Good day tobym;

Is this happening in Internet Explorer or all browsers, perhaps if you are experiencing this in IE 

then there is an IE setting that is disallowing this as per:

https://social.technet.microsoft.com/Forums/office/en-US/0a375bef-9617-488a-bcc5-39cf5ddb724e/page-viewer-is-blank-in-ie?forum=sharepointgeneralprevious

Some possible reasons from this article includes:

1. you access Sharepoint site over HTTPS, while references side (K2) is opened via HTTP or vise versa
* https://help.k2.com/onlinehelp/K2blackpearl/ICG/4.7/default.htm#Prepare/SP/SP_Security.htm?Highlight=Supported Scheme Configurations

2. external web site uses cookies which are blocked in IE. It can be fixed by adding P3P header in the response from external site, see Cookie blocked/not saved in IFRAME in Internet Explorer

3. check if it is related to IE security settings to prevent the cross site scripting:
https://blogs.technet.microsoft.com/srd/2010/04/29/sharepoint-xss-issue/

https://blogs.msdn.microsoft.com/how24/2013/05/28/cross-site-scripting-sharepoint-apps-app-parts-and-office-365/

4. Checking the current IE Document mode, Intranet/Trusted Zones, Compatibility Settings

Best regards,

Dumisani