Actually as a coincidence, it happened to me today, but the culprit was a function from my theme. My homemade theme has a script that displays the page load time, and amount of RAM WordPress consumes, and this displays at the bottom of the footer, both on the front end and back end. While this function was turned on from the Customizer, Gutenberg refuses to save (publish) posts/pages, not even in any mode (publish, draft or preview). Turning it off, everything works fine.
Therefore, since you don’t have any other plugins installed (besides ACF Pro), try changing the theme, to one of the default ones maybe, and see if that fixes your issue. If not, the only advice I can give you, is to save your permalinks again (just in case, although I don’t think this is relevant here), and clear your browser cache, just in case there is some script from the cache causing this error.
Good luck. BTW, my testing environments:
1. Xampp WP 5.0.1 with bunch of plugins, ACF made blocks, on Windows 7 Pro machine, with 4GB RAM, 128MB available for WordPress, php 7.2
2. Godaddy Shared Hosting – php 5.6, and I want to believe 64MB RAM (I can be wrong for the RAM!).
