As weird as it may sound, a Windows Update actually broke the default installation of DotNetNuke. The error occurs when you try to open the default editor page by clicking on any HTML module on the "Edit page" item. You'll receive the following error: "The Runat attribute must have the value Server".
The problem is caused by a file called TextEditor.ascx, located in the folder "Controls" in the root of the DNN application. In this file, some of the quotes are of an incorrect format:
<td id=”celTextEditor” Runat=”server”>
The following is the correct format:
<td id="celTextEditor" Runat="server">
If you change this, your project will compile again like it should!
Because this was caused by an update, already deployed applications will have this problem too.