Yes, the original report 17419 was flagged as a duplicate of 17476 and closed, and 17476 would appear to have been deleted.

I've reposted the bug as 17584, so let's see what happens this time.

I looked at the issue and it seems to be a side effect of a fix we implemented for a different issue (http://support.dotnetnuke.com/issue/ViewIssue.aspx?id=15425&PROJID=2). There is an easy workaround which would fix this issue but bring back the other issue this fix was for.
You need to do is backup \js\dnn.controls.dnnmenu.js from the root of your site (or from a 5.5 package) and replace it after the upgrade.

please make sure your browser's cache is clear after the file is replaced on the server.

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DotNetNuke Corp.
http://www.nonserializable.com/

Interim fix:

Modify dnn.controls.dnnmenu.js and add the highlighted line:

this.renderNode(this.rootNode.childNodes(i),this.container);
this.addHandlers(document.body,{"click":this._bodyClick},this);
if(dnn.dom.browser.isType(dnn.dom.browser.Mozilla,dnn.dom.browser.Safari)==false)
dnn.setVar(this.ns+'_json','');},renderNode:function(node,ctr)
{var mNode=new dnn.controls.DNNMenuNode(node);if(mNode.selected)
this.selMNode=mNode;var menuBuilder=this._getMenuBuilder(mNode,ctr);if(menuBuilder.alreadyRendered==false)