On Firefox https://de.wikipedia.org/wiki/Spezial:Verschieben/Wikipedia:Spielwiese?debug=true throws
Error: Widget not found: wpNewTitle
on https://de.wikipedia.org/w/resources/lib/oojs-ui/oojs-ui-core.js?c7da5 line 691.
Maybe related: The gadget https://de.wikipedia.org/wiki/MediaWiki:Gadget-old-movepage.js loads the module 'mediawiki.special.movePage'.
I can reproduce the second error on a local installation. Therefor I created the separate task T162667.
I first error is related on the gadget because it loades the module 'mediawiki.special.movePage'. The module should be more robust so it does not throw an error.
I don't think this is a MediaWiki bug. 'mediawiki.special.movePage' indeed requires the #wpNewTitle element to be present on the page, but it can do that, because that element is normally always present. I don't actually see how this problem could be happening, hm…
In https://de.wikipedia.org/wiki/MediaWiki_Diskussion:Gadget-old-movepage.js#Gadget_funktioniert_nicht I created a variant of the gadget which loads the module only when the elements are present. This should avoid the error of this task. Of course the gadget can still improved.
Invalid. Not a bug in MediaWiki.