Conflict between GP Premium and reusable blocks

I would say it could be configuration, but that wouldn't be causing a 503 error.

What happens if you create a fresh WP installation on the same server and install GP Premium - same issue?

Creating a new clean install of wordpress and the problem didn't occur, until I imported the posts and pages through the native wordpress export/import tool.
When I did the import, the same error occurred.

And this import is the same set of posts as the last site? What if you import a set of dummy posts/pages? For example: https://codex.wordpress.org/Theme_Unit_Test