I took a SWAG at it and thought the forward slashes weren’t being properly escaped. It appears I was correct.
If the src url includes a ‘/’ (forward slash) you need to double them up (to ‘//’).
So, when I changed my src to
https://www.appsumo.com//ad//?r=ge32&gid=WzEsIDMyNTYyLCAiMjAxMTEwMjAwODUzIiwgImFkIiwgImRhc2giXQ%3D%3D
It worked fine.
So, were do I post bugs for this plugin?