Here is an answer: https://wordpress.stackexchange.com/questions/325570/wordpress-v5-0-3-gutenberg-js-error-uncaught-syntaxerror-missing-after-arg

The simplest way: Find & delete PHP Hook which added 'defer' to script (in the function.php file). Or if you have skills you can edit the code of the hook. Reason: Confusion/conflict with quotes (' & ") witch break down JS.

About 'defer' you can read here: https://www.w3schools.com/tags/att_script_defer.asp This is probably due to speed optimization on your website.