It doesn’t look like it’s the theme. I just did a test with that URL you gave and here is an error I got:
about:blank:1 Refused to display 'https://www.arborday.org/webtools/hortzones/dbRegistration.cfm' in a frame because an ancestor violates the following Content Security Policy directive: "frame-ancestors 'self' www.logees.com logees.com treesandutilities.com www.treesandutilities.com *.secureserver.net *.akam.net *.godaddy.com *.silkroad.com".
I also went directly to that page you gave (not sure what this is for, but got this:
The web site you are accessing has experienced an unexpected error.
Please contact the website administrator.
I tried a couple other commercial sites and they would give security errors. I then tried my blog site with this code in the page I created (in the HTML code view:
<iframe width="600" height="600" src="https://www.andrejutras.ca/" frameborder="0" allowfullscreen></iframe>
It worked. So it appears you are experiencing security issues on the other website(s) that are setup. You may want to contact that website and request information on how to embed the registration form.