Kentico v13.0.151
Error message showing up on page builder even though page builder seems to be working correctly.
This page does not work properly as your browser may be blocking 3rd party cookies and/or the system may be incorrectly configured for a multi-domain environment. See our documentation for more information.
So the iframe can load in the presentation site and I can work with it fine.
3rd party cookies are allowed.
I've tried all the various cookie settings as per the docs.
The 2 apps are running on separate domains with same site cookies set to none and secure as you can see here
Just adding here the answer based on the comments - applying the latest hotfix, following the instructions to the degree fixed the issue.