A 500 error is from the web server itself (before it gets to XenForo). You may have to contact your host or server admin about this. Ideally the error logs on the server itself will have more information. For example, it could be an overly restrictive module like mod_security.
Debug mode didn't give me any info that tells me what the error may be...
My error log doesn't have any errors relevant to this matter...
And my RAW Access log doesn't have any indication that there is an error...
I'm thinking its got to be something with the PHP, but not sure....
This is from the raw access log, about the error:
[28/Mar/2014:10:57:36 -0400] "GET /admin.php?users/moderated HTTP/1.1" 500 546 "http://www.autoracinghq.com/admin.php?users/" "Mozilla/5.0 (Windows NT 6.3; WOW64; Trident/7.0; Touch; rv:11.0) like Gecko"
As Jake said, typically those errors occur before XenForo code starts executing. So, yes, it is more likely to be related to Apache or PHP and therefore it is those error logs you should be interrogating.
However, could you clarify a little more about the issue?
How many users do you have awaiting approval?
Are you able to see the list of users, or does it not get that far? Or is it when you process the selected users. If the latter, have you tried Mike's suggestion of processing less users at a time? e.g. does it work if you, say, delete or approve only 2 or 3 users?
What add-ons do you have installed, and do any of them modify anything to do with users awaiting approval? Easiest way to check this is to add $config['enableListeners'] = false; to your config.php file and then try again.
Mike also mentions possible server restrictions. Does your server have anything like mod_security enabled? If so are there any related logs that contain any information about this error?
I have 43 users awaiting approval...
I can see my user list when I select all users.., And I can delete users from the user list.
The only page showing a 500 error is when I click on show users awaiting approval.
I don't have any add ons enabled.
I just upgraded to 3.0 from 2.3
I do have mod_security and there are no errors
Are you in a position to be able to disable mod_security, just to eliminate that being an issue? It's odd if it hasn't logged anything, but as you have described, the behaviour doesn't seem consistent. e.g. If you're loading your full user list you would expect similar issues if it was related to memory or some other limits, so it still seems to be something external to XenForo. And in the absence of any other logs, disabling mod_security temporarily might be a good step.