I had a problem with rendering a JavaScript / iFrame in the admin view of a block.
What you can do is change the admin view so that you don’t see the complete realtime rendered block but for example only an image or a small text.
This is the solution in most cases.
I had the same issue which started with a JavaScript error.
In my case I had a problem with a Custom ACF Gutenberg block that was in preview mode.
When you switch all the blocks to edit mode, then save the page and reload the page there should be no JavaScript error.