I tested with BE 1.6 and couldn't reproduce :(
I don't think the storage location matters (DB, XML), as you noticed.
In the WidgetEditor.aspx page in the ADMIN folder, does the top line include ValidateRequest="false" ?
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="WidgetEditor.aspx.cs" Inherits="User_controls_WidgetEditor"
The ValidateRequest part was even in prior versions too. So I'm guessing you probably have that part.
Sorry, right now that's all that's coming to mind .....