Looks like the node you are passing in has become disconnected from the document.

In T202714#4550942, @Esanders wrote:

Looks like the node you are passing in has become disconnected from the document.

Yes, when default MT provider fails, we fallback to adapting source and the passed node is of type ve.dm.CXPlaceholderNode, which is detached from the document.

I have tried translating (from German to English) the articles with similar infoboxes to provide more examples where this error is happening and how is rest of the translation process affected.
Similar articles about lakes, de:Klostersee_(Lehnin) and de:Heiliger_See_(Potsdam), as well as article about hill de:Kutschenberg are throwing the same error, which prevents user from translating the infobox. Translation of other sections is not affected.

On the other side, articles with similar infoboxes, de:Sanssouci (about palace) and de:Talsperre_Spremberg (about dam) are not affected by this JS error. Infoboxes of these articles are translatable.

Is there a test server where I can test this?

In T202714#4574086, @Esanders wrote:

Is there a test server where I can test this?

You can test here. Just try translating the infobox.

So the problem is you completely replace the node in the first attempt at translation, so by the time it falls through, the node no-longer exists.

You can fix the replaced node by using the same technique you using in the .always block, so in onDocumentActivatePlaceholder:

model.emit( 'beforeTranslation' );
this.MTManager.getPreferredProvider().then( function ( provider ) {
	return this.changeContentSource( model, null, provider );
}.bind( this ) ).fail( function () {
	// If placeholder has been destroyed, look for a section node
	if ( !model ) {
		model = this.getTargetSectionNode( cxid );
	}

however this doesn't fully fix the issue, as the translation fails even with source="source", so the second 'changeContentSource' also fails, and isn't handled. You'll need to add more error handling to leave the document in a sensible state.