Block editor completely gone for second of two super user

tl;dr: Two super users in a blog network. 15 sites. When second user logs-in block editor is gone. I can see Gutenberg > Demo menu item, I can open “Welcome to Gutenberg Editor” demo page, but this page (as well as all other pages and posts in all 15 sites in entire blog network) is rendered using regular code editor. For first user everything works as expected.

I have a blog network with 15 sites. I am using Gutenberg editor for editing my posts on every of these sites. I have Gutenberg plugin installed separately. Auto-updates enabled. Now version 9.3.0.

I also have two users in my blog network and they are both Super Users:

But the second one is actually inactive, used once per year or less. I post most of my posts via first user.

Gutenberg is installed, network activated and available on each site without any problems:

When I login as first user (admin, the one that I use the most) then everything is OK:

When I login with second user, on each and every site Gutenberg is simply… gone:

As you can see in linked images, when I am logged-in as second user, I can still see that plugin is enabled and available, I can still see “Gutenberg” option in my side bar, I can still even click Gutenberg -> Demo, but when I click it… there’s no Guteberg at all. The same for all posts and pages and for each and every site.

Tested on PC, under newest version of Chrome and Edge under Windows 10 and on mobile phone under newest version of Chrome available for Android 8.0 Oreo. Tested on each and every of 15 pages that are part of my blog network. Everywhere exactly the same effects. All is working fine and dandy for the first user + Gutenberg is completely gone for the second user.

For a short period of time I have enabled SCRIPT_DEBUG mode as instructed here and here, but DevTools console under Microsoft Edge showed nothing alerting:

So… nothing unusual, if I am not mistaken.

This is as weird as it can be and I am completely lost on where to even start looking for any source of this issue or any solution or workaround for it. Please, help, if you can.