I selected all VE content from: "It is important to make", up to "Mediawiki-l mailinglist thread "mysql5 binary schema" 25.03.2009"
I could pinpoint it to trying to copying the Header "scripts" and the warning template that follows it.
[Error] TypeError: 'undefined' is not an object (evaluating 'delete children[i].values[attribute]')
removeAttributeRecursive (load.php, line 20)
removeAttributeRecursive (load.php, line 20)
removeHtmlAttribute (load.php, line 20)
getClonedElement (load.php, line 202)
cloneElements (load.php, line 170)
onCopy (load.php, line 292)
proxy (load.php, line 10)
dispatch (load.php, line 45)
eventHandle (load.php, line 38)
Version: unspecified
Severity: normal
URL: https://www.mediawiki.org/wiki/Manual:Backing_up_a_wiki?veaction=edit