In T268953#6661021, @matmarex wrote:

Assuming you're using Apache, can you try adding AllowEncodedSlashes NoDecode to the virtual host configuration? (This is documented at https://www.mediawiki.org/wiki/Extension:VisualEditor#Troubleshooting, but under a slightly different error message than you're getting, which is interesting.)

For me, it fails when opening the page for editing.

I assume WFM means "works for me". If needed I can provide additional information. I just couldn't find anything relating to the error. The API response had very little information in it.

In T268953#6654452, @Reedy wrote:

And for clarity... Editing non subpage pages actually work?

That worked.

I updated the description with some crude investigative work.

@Esanders this is one of 3 errors I have found (so far) that blocks devs from setting up VE properly (at least, if they do so by following the install instructions on the wiki).

I solved this by following this discussion on the Wiki talk page.