@sinja @Jacalz this is an interesting one.
I have started taking a closer look, and have tested some workarounds while I investigate further.
Right now, the only shield setting preventing this newsletter from displaying is the ad blocking shield.
I have tested with cookie controls off/on, https off/on, and with ad blocking on and off. It turns out that ad blocking is the switch that prevents this from rendering.
Disabling the ad blocking setting should resolve short term.
I took a closer look at the source code, and it looks like Ello is treating the content as if it was an ad for it to work with the mailchimp newsletter system. I need to see a little more as to why, but that is what I am able to see so far.
I am going to check against other blockers to see if we can set an exception filter to allow for this to work without manually adjusting in the long term. Will open an issue and update with a link here for tracking progress.