I'm writing this report in the assumption that Visual Editor is supposed to work 'out-of-the-box' which it doesn't for me - i actually didn't get it to work at all yet.
Coming from here https://www.mediawiki.org/wiki/Topic:Vunt92vkiyisn8cu others seem to have the same issues.
I'm maintaining a small Wiki-Family with a global wiki only for images in the root folder (hidden to users, they don't get there without knowing direct links) and localized wikis in subfolders (/de, /en, etc.).
This done with a basic symlink setup as described on the respective Wiki-Family mediawiki page and works fine.
Out-of-the-box with that setup the Visual Editor errors out with 'Error contacting the Parsoid/RESTBase server (HTTP 404)' on all localized wikis in subfolders but in the global wiki in the root.
Configuring manually the following URLs for each family wiki will remove that error.
$wgVisualEditorRestbaseURL = "https://example.com/en/api/rest_v1/page/html/";
$wgVisualEditorFullRestbaseURL = "https://example.com/en/api/rest_";
(Replacing /en with the respective subfolder, of course)
Unfortunately the Visual Editor is nontheless not working on any of the wikis, neither the global one in the root nor the family wikis. When attempting to edit a page i do see the Wiki Editor, but the actual content of the page is gone and the loading icon is persistent as in the picture below that should technically show a lot of headlines and text, as the page is full of it.
To make sure this is not caused by family setup i've installed a new mediawiki on a dedicated subdomain separate from all other wikis and it shows the same picture like above (while not showing the 404).
I'm possibly just too dumb to use, but given it's supposed to work out-of-the-box i thought to report this as noone else in the mentioned thread has done it yet.