Going through DEBUG logs - What am I looking for?

I was working on the Admin of my new site, adding products and categories. When I go to add categories, I get an Error 500 message. So I've found all my logs, but I have no idea what I'm looking for. And when I find this whatever-it-is, what am I supposed to do with it?

(Background - This isn't my first site, and over the past 7 years, I've learned to do some coding work on my own with the help of a couple of friends. However, I'm far from proficient. My web developer was creating this new site for me, but it's like he's dropped off the face of the earth, and my new site was supposed to be up this past Monday.)

You say you have a 500 error and that you have found "the logs". Which logs have you found?

Generally speaking a 500 error is something caused by a problem on the server; however, there are some 500 errors for which ZC will be able to generate an error log (myDebugxxxxx). ZC error logs can be found in the logs directory of your store. The server logs are different and you would be looking to see what errors were generated when you follow your own traffic (ip address) up to the point of when the error occurred and try to identify what the logs say.

If the logs you are looking at are the ones generated by ZC, then look at the most recent. You're welcome to post it here, but please be sure to obscure your admin directory name (replace the text with admin for example) before posting. Considering that other aspects of admin navigation seem to be working, it would seem that the issue is relatively narrow focused, which can be good...

Might also suggest navigating to other similar links from the menu(s) and see if anything else provides a 500 error.

More about '500' errors: https://www.zen-cart.com/content.php?105-why-am-i-getting-500-internal-server-error

But ... post the error messages here so we can give feedback. Chances are the errors you're seeing are already reported before too ;)

This is the error message in the most recent log entry, dated a few days ago...
[04-Jun-2016 18:59:39 America/Denver] PHP Fatal error: Class 'notifier' not found in /home/***/public_html/includes/autoload_func.php on line 79.

I got up with a friend who's not well-versed in ZC but who suggested I check my cPanel files for a missing or corrupt file.

autoload_func.php loops through the list of files it's told to load as declared in the /includes/auto_loaders/config.core.php file. Some of the instructions there are to load certain files. And some are to execute code found within those files.

In your case if it's saying it can't find the "notifier" class, then that means that it didn't properly load the /includes/classes/class.base.php and/or class.notifier.php files ... perhaps because they're missing or damaged. Or (in rare cases) their file-permissions is set so low that PHP isn't allowed access.

So, yes, chances are that you uploaded a large number of files but some failed for some reason and you didn't get notified of the failures via your FTP program.

I found a huge block of folders (dozens) that I missed in the update. In fact, I'm still working on those. I'm hoping this works. I trust there's a place to find the files I'm missing if it doesn't. I haven't taken the time to look, yet, because I'm trying to get these files/folders uploaded.

I appreciate the "why" in addition to the "what."

I think this worked. I went through and installed all the files I'd missed in the update. When I tested it out, I didn't get the error message. (Yea!!!) My second problem to deal with was, when I typed in my url, I'd get my admin login page. Now I get just a blank page, which is slightly better. So, hours of work paid off. Thanks, folks, for your help on this problem! :)

The logs will usually help explain the blank page too.