I run into the same problem after having installed server certificates over LetsEncrypt: IframeWrapper links ended in an empty page.
I tried the IframeWrapper option
Metadata / Secure from IGNORE to ON or OFF - nothing changed.
Looking at the source code of the Iframe with firefox developer tools, I noticed, that the Iframe link after each change of the avove options started with http. After manually changing it in the source code tho https, the Iframe content suddenly appeared.
This helped me to find the very simple solution of the problem:
In Joomla Backend go to the properties of each IframeWrapper menu link and check the option
Advanced / Auto Add
from YES to NO