Is the categories plugin installed?
Is there are block like

		<?php
			// -------------------------- CALENDAR INCLUDED HERE -----------------------------
			// Call the Calendar plugin:
			$Plugins->call_by_code( 'evo_Calr', array(	// Params follow:
					'block_start'=>'',
					'block_end'=>'',
					'title'=>'',			// No title.
				) );
			// -------------------------------- END OF CALENDAR ----------------------------------
		?>

in your skin's _main.php file?

That following section of code reads as follows:

<?php
		// -------------------------- CATEGORIES INCLUDED HERE -----------------------------
		// Call the Categories plugin:
		$Plugins->call_by_code( 'evo_Cats', array(	// Add parameters below:
			) );
		// -------------------------------- END OF CATEGORIES ----------------------------------
	?>

The categories plugin is installed and enabled to the best of my knowledge. It appears under the plugin tab and isn't marked as disabled.

The plugin seems to get called, yes - there's the "Categories" title on your blog.

So there seems to be a problem/bug with the code of the categories plugin.

You will have to look into the SkinTag function in /plugins/_categories.plugin.php and add debug output, e.g. "pre_dump($blog);" or "pre_dump($tmp_disp);" in the appropriate places.
You should add a "if( $_SERVER['REMOTE_ADDR'] == 'YO.UR.IP.ADDR' ) { ... }" around it, so only you see this.

This may be "too much" for you, so you might want to send me a ftp account (E-Mail or PM) and I will look at it.

I'll make the changes and take a look as soon as I get the chance. Thanks for the continuing assistance. I'll post again as soon as I can get the changes made and can look at the bedug output.

I'm unsure of where to add the "pre_dump(...)" code?

Also, just for my own edification, where is the $params variable(array?) set?

You would add e.g. pre_dump() at the beginning of the SkinTag function - after the "global $blog" line. To know what $blog is. $params get passed to SkinTag. It's the array ("...") in the skin's $Plugins->call_by_code( 'evo_Cats', ... ) call.

Still: if you have problems debugging this yourself, please just create an FTP account for me and I'll take a look.

I added pre_dump($blog); to the end of the line with "global $blog" as shown below:

function SkinTag( $params )
	{
	 	global $cache_categories;
		/**
		 * @todo get rid of these globals:
		 */
		global $blog, $cat_modifier; pre_dump($blog);

My blog page now has: "int(1)" just above the categories section. Does this mean anything to anyone?

Should I add "pre_dump($blog);" elsewhere?

$blog == 1 now means that it goes into the second if-block ("else { // We want to display cats for all blogs")

From looking into this, I now guess that you have unchecked the "display in public bloglist" option of your "Blog 1". So it gets skipped. You can change this in the blog settings.

To be sure, add "pre_dump($curr_blog_ID);" in a line just before "// run recursively through the cats" (~ line 192). My guess is that there will nothing get outputted, because of the "continue" before this.

If this should not be the case, I'm sorry and you will have to find the reason yourself.

I now guess that you have unchecked the "display in public bloglist" option of your "Blog 1".

This was the problem. Thanks for all the help.