When a new wiki is created, it takes some time before RESTBase is set up for it. A lot of things depend on RESTBase, and don't work without it. For all the things listed below, it seems like RESTBase is a hard dependency (at least in Wikimedia's setup), but the features still initially give the *appearance* of working, even though they don't.
All the screenshots are from https://anp.wikipedia.org/ with the interface language set to English. It was set up in T332115 six days ago; six days may not seem like much, but it is a long time to live with these errors for a community that has finally gotten its brand new wiki. For anpwiki's case, this will all be solved when T332380 is solved, but this keeps happening for every new wiki. Some times RESTBase is set up quite quickly after wiki creation, but some times it can take several days (like now).
Features with problems
Visual editor
Without RESTBase, you can start editing a new page with Visual Editor. However, when you try to save that page, it gives an error:
Likewise, you get an error when you try to edit an existing page, but then at least the error appears when you *start* editing:
Visual diffs
Visual diffs just show nothing. Here's a comparison between a visual diff and the same diff in wikitext:
Discussion tools
For the discussion tools, you can't actually post replies. It lets you start *writing* a reply, but you can't save it. It is also impossible to switch from source mode to visual mode.
Reply:
Curiously, you can create new topics, but only if you use the source editor. If you use the visual editor, you get the same error as above (as expected, really):
Content Translation
When you save a translation with the Content Translation tool, all of your text is lost and instead it saves some JSON as wikitext (!). More details about that in T333238.
Proposed solution
Because RESTBase is seemingly a hard dependency for these features, the features themselves should be temporarily disabled as long as RESTBase is not set up for a wiki. The current state, where contributors can start edits but aren't able to save them is potentially harmful since it wastes contributors' time and efforts.