When the template has some bad HTML syntax Page Builder will fail with a very general error.

Steps to reproduce:
1. Change default block preview layout - add two classes to div tag:

<div class="landing-page__block block_{{ block.type }} first-dummy-class  second-dummy-class" data-ez-block-id="{{ block.id }}">

Please notice two spaces between first-dummy-class and second-dummy-class.
2. Open landing page in Page Builder. Click on Edit.
3. Drag Code Block to page.

Result:
Adding new block will fail with very hard to debug error:
Uncaught DOMException: Failed to execute 'add' on 'DOMTokenList': The token provided must not be empty.