Some quick findings: looking through Chrome's developer tool before I toggle the rich editor in the Edit page, I get a warning:
- load.php?debug=false&lang=en&modules=jquery%2Cmediawiki&only=scripts&skin=vector&version=yk8V6aOR:156 Use of "CKEDITOR_BASEPATH" is deprecated. Use mw.config instead.
After I toggle the rich editor I get some potentially interesting errors:
load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:37 GET https://ourdomain.tld/wiki/Test/extensions/WYSIWYG/ckeditor/config.js?t=G4CD
append @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:37
appendTo @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:25
t @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:110
load @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:110
a @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:110
CKEDITOR.scriptLoader.queue @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:110
l @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:116
k @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:117
(anonymous) @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:115
(anonymous) @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:12
load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:37 GET https://ourdomain.tld/wiki/Test/extensions/WYSIWYG/ckeditor/lang/en.js?t=G4CD
append @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:37
appendTo @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:25
t @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:110
load @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:110
load @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:109
w @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:117
(anonymous) @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:117
b @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:234
loadPart @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:234
q @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:117
(anonymous) @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:117
q @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:3
CKEDITOR.event.CKEDITOR.event.fire @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:4
CKEDITOR.editor.CKEDITOR.editor.fire @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:5
fireOnce @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:4
CKEDITOR.editor.CKEDITOR.editor.fireOnce @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:5
l @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:116
(anonymous) @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:116
callback @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:110
f @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:109
y @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:109
A @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:110
CKEDITOR.env.ie.CKEDITOR.env.version.f.$.onerror @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:110
load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:109
Uncaught TypeError: Cannot set property 'dir' of undefined
at Object.d (load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:109)
at f (load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:109)
at Array.y (load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:109)
at A (load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:110)
at HTMLScriptElement.CKEDITOR.env.ie.CKEDITOR.env.version.f.$.onerror (load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:110)
d @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:109
f @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:109
y @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:109
A @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:110
CKEDITOR.env.ie.CKEDITOR.env.version.f.$.onerror @ load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:110
load.php?debug=false&lang=en&modules=ext.CKEDITOR|ext.WYSIWYG.func&skin=vector&version=c049238de9d8:37 GET https://ourdomain.tld/wiki/Test/extensions/WYSIWYG/ckeditor/skins/kama/editor.css?t=G4CD
@Riparap, does this help to narrow down the problem?