We found that uBlock Origin (version 1.17.4) is preventing embedded Marketo forms from loading.
Also, when editing an email in Marketo Designer, uBlock Origin causes a system error (an error message pops up).
Adding marketodesigner.com to the uBlock Origin whitelist is a workaround.
Prior to today, Marketo was not listed by uBlock Origin.
It's clear that you can't expect every site to operate when you literally have a site-blocking plugin installed!
For the Designer, just whitelist it.
For the form, load the embed from your LP domain, as I mentioned in your other thread.