1. It has nothing to do with admin.php
2. Nothing was changed on the server when upgrading

The "link" that doesn’t work in my Mystique skin is http://www.tylerhosting.com/b2e/skins/mystique_skin/#instance-sidebartabswidget-section-categories, which was probably handled by php.
(Actually all four in the header fail, but with different URLs)

Come to think of it, this is just one aspect of the problem though it may be a hint to what to look for. The categories are supposed to display by default and they don’t, and then selecting the icon that does that shows this error.

Anyway, here’s what I tracked down so far.

There is a style in the skin’s styles.css with the id

instance-sidebartabswidget-section-categories

a. The skin php that refers to that "URL" is in the skin’s _skin.class.php in the function display_tabbed_widget()

b. The first line in that function is
if( ! $this->get_setting( ‘tabbed_widget’ ) ) return;
which I’m assuming is what sets the text (or in this case fails to) for that selection

c. I couldn’t find a function get_setting() that handles the parameter ‘tabbed widget’, but did find the function get_param_definitions() that does handle that parameter ‘tabbed widget’ in the skin’s file _skin.class.php

d. I found multiple files, in the skin and in the b2e code that calls get_param_definitions() so that was my deadend.

What sections process skin content? I do have the code for the prior version, so I may compare to see what is missing/different if there’s not too much to go through.
I really liked that skin, but if I can’t fix it I guess I’ll have to select another.