Weird problem with Gutenberg partially loading (not loading blocks)

Hello,

I have a weird problem with one of the sites I’m currently supporting. It’s an old website installed on 2014 and updated more or less regularly, having few themes and plugins active through this time, oriented on blogging (blog + podcast + some analytics + wpml + commercial theme with its addons).

Everything is up to date (wordpress, plugins, theme).

Right now, whenever an edit screen is opened (post, page or cpt), Gutenberg seems to load fine – top bar, sidebar and central content seem ok. However, it doesn’t load blocks – there’s just one main content area with HTML wrapped with Gutenberg comments (when editing existing posts) ie.:

<!-- wp:separator --> or <!-- wp:more -->

It basically looks like this:
https://www.dropbox.com/s/ijpwlxk0hlv2wrg/gutenberg-error.png?dl=0

Important bits:
a) disabling all plugins and using default theme doesn’t fix the issue
b) migrating the site to localhost and doing the above doesn’t fix the issue
c) rest api is available via /wp-json
d) clearing browser cache doesn’t help
e) it happens both in chrome and firefox (latest stable)

What I’ve also found:
On a test copy (on a subdomain) when editor is loading, I can see an error in the console:

Error while traversing the CSS: Error: undefined:11:1: missing '{' block-editor.min.js:49:219418

While I don’t see this error on production version or localhost, it might be related to the problem. Overwriting this file (or all files) from fresh wordpress archive doesn’t fix the problem.

Also, running wordpress with:
define( 'SCRIPT_DEBUG', true );
does produce extra warnings in the console from react-dom.js regarding using strict mode but the problem persists.

Any support will be greatly appreciated – what to look for, how to test things, etc.