Navigation broken in Sitecore Page Editor during Edit related item - editor

Question

After updating Sitecore 6.3 to 6.6, the Main Navigation is broken in the Sitecore Page Editor when "Edit related item" is clicked.
It looks like this:
It even stays like this when closing the "Edit related item" frame.
Here is the text from that screen:
{"commands":[{"click":"chrome:common:edititem({command:\"webedit:open\"})","header":"","icon":"/~/icon/SoftwareV2/16x16/cubes_blue.png.aspx","isDivider":false,"tooltip":"Dieses Item im Content Editor bearbeiten.","type":"common"}],"contextItemUri":"sitecore://master/{78EBD797-ACA9-40BC-9ACE-961CD2296CDC}?lang=de-CH&ver=1","custom":{},"displayName":"Title","expandedDisplayName":null}KATA {"commands":[{"click":"chrome:common:edititem({command:\"webedit:open\"})","header":"","icon":"/~/icon/SoftwareV2/16x16/cubes_blue.png.aspx","isDivider":false,"tooltip":"Dieses Item im Content Editor bearbeiten.","type":"common"}],"contextItemUri":"sitecore://master/{F195AD36-86EE-4C13-883B-761E300C23AF}?lang=de-CH&ver=1","custom":{},"displayName":"Title","expandedDisplayName":null}REA {"commands":[{"click":"chrome:common:edititem({command:\"webedit:open\"})","header":"","icon":"/~/icon/SoftwareV2/16x16/cubes_blue.png.aspx","isDivider":false,"tooltip":"Dieses Item im Content Editor bearbeiten.","type":"common"}],"contextItemUri":"sitecore://master/{63438A98-FC6F-461D-89BB-7497B12FBAEF}?lang=de-CH&ver=1","custom":{},"displayName":"Title","expandedDisplayName":null}Patientensicherheit {"commands":[{"click":"chrome:common:edititem({command:\"webedit:open\"})","header":"","icon":"/~/icon/SoftwareV2/16x16/cubes_blue.png.aspx","isDivider":false,"tooltip":"Dieses Item im Content Editor bearbeiten.","type":"common"}],"contextItemUri":"sitecore://master/{FB9B0590-E933-4141-BA2B-F82C83A3343E}?lang=de-CH&ver=1","custom":{},"displayName":"Title","expandedDisplayName":null}Prozesse {"commands":[{"click":"chrome:common:edititem({command:\"webedit:open\"})","header":"","icon":"/~/icon/SoftwareV2/16x16/cubes_blue.png.aspx","isDivider":false,"tooltip":"Dieses Item im Content Editor bearbeiten.","type":"common"}],"contextItemUri":"sitecore://master/{0F1334E0-9BB7-4657-9DC4-884F6E1133C9}?lang=de-CH&ver=1","custom":{},"displayName":"Title","expandedDisplayName":null}Dokumente {"commands":[{"click":"chrome:common:edititem({command:\"webedit:open\"})","header":"","icon":"/~/icon/SoftwareV2/16x16/cubes_blue.png.aspx","isDivider":false,"tooltip":"Dieses Item im Content Editor bearbeiten.","type":"common"}],"contextItemUri":"sitecore://master/{8D18489B-B7F3-442B-9958-7D1FDBDC9010}?lang=de-CH&ver=1","custom":{},"displayName":"Title","expandedDisplayName":null}Telefonlisten
Anyone knows what this is and how I can fix it?
Thanks in advance :)
EDIT:
Code of .ascx:
<%# Control Language="C#" AutoEventWireup="true"
CodeBehind="Header.ascx.cs" Inherits="ZGKS.Intranet.WebApp.Frontend.sublayouts.Header" %>
<%# Register Src="NavLayer.ascx" TagName="NavLayer" TagPrefix="uc1" %>
<%# Register Src="~/Frontend/sublayouts/SearchBox.ascx" TagName="SearchBox" TagPrefix="ZGKSControls" %>
<div class="head">
<div class="line">
<div class="unit sizeCol">
<!-- mod NavService -->
<div class="mod modNavService skinNavServiceHome">
<div class="inner">
<div class="bd">
<ul>
<li>Home</li>
<li><span>
<asp:Literal ID="litWelcome" runat="server"></asp:Literal>
<asp:Literal ID="litUsername" runat="server"></asp:Literal></span></li>
<li class="last"><span>
<asp:Literal ID="litDate" runat="server"></asp:Literal></span></li>
</ul>
</div>
</div>
</div>
<!-- /mod NavService -->
<!-- mod NavService -->
<div class="mod modNavService">
<div class="inner">
<!-- tpl NavService-navservice -->
<div class="bd">
<ul>
<asp:Literal ID="litNaviLinks" runat="server"></asp:Literal>
</ul>
</div>
<!-- /tpl NavService-navservice -->
</div>
</div>
<!-- /mod NavService -->
<!-- mod Favourites -->
<div class="mod modFavourites">
<div class="inner">
<!-- tpl Favourites-favourites -->
<div class="hd">
<ul>
<li>Meine Favoriten
<div class="favnavcontainer">
<div class="wrap">
<div class="list">
<table>
<tbody>
<tr>
<td>
<asp:Label ID="lblFavoritesDescription" runat="server" />
</td>
</tr>
</tbody>
</table>
<table class="highlight">
<tbody>
<asp:Repeater ID="rptFavorite" runat="server" OnItemCommand="RptFavoriteItemCommand">
<ItemTemplate>
<tr class="favoritemark">
<td>
<a href="<%# Eval("Url") %>">
<%# Eval("Title") %></a>
</td>
<td class="delete">
<span>
<asp:ImageButton ToolTip="Löschen" AlternateText="Löschen" ID="btnDeleteFav" runat="server"
CommandName="delFav" CommandArgument='<%# Eval("ID") %>' ImageUrl="~/Frontend/Images/icons/clear.png" />
</span>
</td>
<td class="edit" title="Editieren">
<span></span>
</td>
</tr>
</ItemTemplate>
</asp:Repeater>
</tbody>
</table>
</div>
</div>
<a class="close" title="" href="#">Schliessen <span>|</span> X</a>
</div>
</li>
<li>Meine Systeme
<div class="favnavcontainer">
<div class="wrap">
<div class="list">
<table>
<tbody>
<tr>
<td>
<asp:Label ID="lblSystemDescription" runat="server" />
</td>
</tr>
</tbody>
</table>
<table class="highlight">
<tbody>
<asp:Repeater ID="rptSysteme" runat="server">
<ItemTemplate>
<tr class="">
<td>
<a target="_blank" href="<%# Eval("Url") %>">
<%# Eval("Title") %></a>
</td>
</tr>
</ItemTemplate>
</asp:Repeater>
</tbody>
</table>
</div>
</div>
<div class="modContent" style="padding: 0;">
<div style="margin: 10px 0; margin-left: 10px;">
<a target="_self" href='<%# MySystemLink %>' class="icon back" title="Zurück">
<span>Meine Systeme bearbeiten</span>
</a>
</div>
<a class="close" title="" href="#">Schliessen <span>|</span> X</a>
</div>
</div>
</li>
<li>Meine Alerts
<div class="favnavcontainer">
<div class="wrap">
<div class="list">
<table>
<tbody>
<tr>
<td>
<asp:Label ID="lblAlertDescription" runat="server" />
</td>
</tr>
</tbody>
</table>
<table class="highlight">
<tbody>
<asp:Repeater ID="rptAlert" runat="server" OnItemCommand="RptAlertItemCommand">
<ItemTemplate>
<tr class="alertmark">
<td>
<a href="<%# Eval("Url") %>">
<%# Eval("Title") %></a>
</td>
<td class="delete">
<span>
<asp:ImageButton ToolTip="Löschen" AlternateText="Löschen" ID="btnDeleteAlert" runat="server"
CommandName="delAlert" CommandArgument='<%# Eval("ID") %>' ImageUrl="~/Frontend/Images/icons/clear.png" />
</span>
</td>
<td class="edit" title="Editieren">
<span> </span>
</td>
</tr>
</ItemTemplate>
</asp:Repeater>
</tbody>
</table>
</div>
</div>
<a class="close" title="" href="#">Schliessen <span>|</span> X</a>
</div>
</li>
</ul>
</div>
<div class="bd">
</div>
<!-- /tpl Favourites-favourites -->
</div>
</div>
<!-- /mod Favourites -->
</div>
<div class="unit sizeCol lastUnit">
<div class="line">
<!-- mod Logo -->
<div class="mod modLogo">
<div class="inner">
<!-- tpl Logo-logo -->
<div class="bd">
<a href="/" title="">
<img class="screen" src="/Frontend/Images/logo.gif" width="291" height="27" alt="ZGKS"></a>
</div>
<!-- /tpl Logo-logo -->
</div>
</div>
</div>
<div class="line">
<ZGKSControls:SearchBox ID="SearchBox" runat="server" />
</div>
</div>
<div class="line">
<div class="unit size1of1">
<uc1:NavLayer ID="NavLayer1" runat="server" />
</div>
</div>
</div>
</div>

Answer 1

The issue is due to changes in the webedit.css stylesheet in Sitecore 6.6 from previous versions.
Make sure that /website/webedit.css is present in your inetpub folder, that you do not have it checked into your Visual Studio project, i.e it is not overwriting with a previous version of the file from your source control.