I just got around to tracking this back.

During the process of upgrading the unit the device will present upgrade61.html.

This code for this site is

Code:
<html>
  <head>
    <script type="text/javascript" src="upgrade61.js"/>
    <link rel="stylesheet" type="text/css" href="images/base.css"/>
  </head>

  <body>
    <center>
      <p>
	<b>Untangle 6.1 upgrade. Please do not reboot the machine, it will automatically do so once the upgrade is complete.</b>
      </p>

      <div class="main-mid" id="upgrade" style="border:solid 1px #222222 ; width:90% ; text-align:left ;">
      </div>

      </center>
  </body>
</html>
Because this document doesn't have a doctype directive, browsers are interpreting the page in quirks mode and thinking this html is 4.01 transitional.

Within that doc type the "short tag" for script is not supported. Due to this bug, the page presents as a blank white sheet to anything other than IE users.

Long and short if you upgrade a 6.02 installation to 6.1 and you use a browser that isn't IE the user NEVER sees the "PLEASE DON'T REBOOT" warning.

The quick fix is to use the following code instead.

Code:
<html>
  <head>
    <script type="text/javascript" src="upgrade61.js"></script>
    <link rel="stylesheet" type="text/css" href="images/base.css"></link>
  </head>

  <body>
    <center>
      <p>
	<b>Untangle 6.1 upgrade. Please do not reboot the machine, it will automatically do so once the upgrade is complete.</b>
      </p>

      <div class="main-mid" id="upgrade" style="border:solid 1px #222222 ; width:90% ; text-align:left ;">
      </div>

      </center>
  </body>
</html>
However, due to browser limitations even if you use XHTML 1.0 strict you can't get away with the short tag on script... it just doesn't work.