Internal Server Error or Error 500

Error 500 is a generic error that can be caused by many things and it gives little or no information about its cause.

Joomla 3.x
Login screen displayed with booking screen.
Joomla changes the URL after login.

If you have a login module displayed with your booking screen, and the user logins in while on the booking screen, the URL changes and can cause an error 500 because the URL is not what it was when the screen was initially rendered.
Do not display a Joomla login module on the same screen as the bookings, or set the login redirect to Home.


## Using Joomla 3.1? See bottom of this page ##

While researching problems with Joomla 3 installation I read Error 500 can be caused by permissions being wrong.
On some(most/all?) servers..

the default file permissions need to be 755 for directories and 644 for files, otherwise you get the Internal Server Error.

There is a handy utility called SiteScan that will scan your Joomla site and set permissions. I have used it to solve the 500 error caused by incorrect permissions on the initial Joomla 3 distribution. I have never encountered an error 500 on ABPro so I cannot say it it will be of use or not.
archive.extensions.joomla.org/extensions...security-tools/12152

Also, if you are using SEO and your htaccess file is re-writing URLS off into never-never land, the server may return a 500 error as it cannot find the code to run.
Try turning off SEO and renaming your .htaccess file back to htaccess.txt. ABPro is tested with Joomla SEO and the supplied htaccess file. Changing the htaccess file can break ABPro.
Use of an SEO plug-in can also cause ABPro server calls to fail. If using a 3rd party SEO plug-in see if you can tell it to ignore ABPro.

I have another customer whose server was returning 500 but in fact the data was being returned just fine. He edited script.js and changed all instances of
if (xhr.status === 200)
to
if (xhr.status === 200 && xhr.status === 500)
and that worked.
Note there are 15 places where the check for 200 is made, not all are xhr.status, if you try this one, look for all
status === 200

Another possibility if you are using ABPro prior to 2.0.4 beta 4 and categories..
Edit file: getSlots.php
Around line 193 look for:
$res_top_row.'\' as description, 0 as ordering, "" as cost, \' \' as access) '.
Change to:
$res_top_row.'\' as description, 0 as ordering, "" as cost, \'|-1|\' as access) '.

See also
appointmentbookingpro.com/component/kune....html?Itemid=0#13566

One customer found the Error 500 only showed on front-end admin screens when SEO enabled without the html suffix option. (I am unable to reproduce that behavior)
His solution..

I have managed to workaround that however, by examining the URL with .html suffix turned off and the URL with it turned on. and then adding a Rewrite condition into the .htaccess to convert the URL generated by Add Booking (with the .html suffix turned off) into the URL generated (when .html suffix is turned on). The following is the redirect:

RewriteCond %{QUERY_STRING} (^|&)task=add_booking($|&)
RewriteCond %{QUERY_STRING} (^|&)frompage=front_desk($|&)
RewriteRule ^online-booking-front-desk/booking_screen_gad$ /index.php?option=com_rsappt_pro2&controller=booking_screen_gad&task=add_booking&frompage=front_desk&Itemid=158 [L,R=301]

Adding that into the .htacess makes it work with SEF URLs turned on but with the .html suffix setting turned off.

I hope the above makes sense and is useful for someone. The ItemID would need to be different depending on the installation, but the rest it should work I think if the correct ItemID is included

Updating your version of ABPro requires an uninstall/re-install, after which you should always reset the ABPro menus, especially the Front Desk one.
In most cases it merely requires you to go into the Joomla menus setup, click on each menu item to edit, then save it again.

The Front Desk sets many parameters in the menu setup and if that is not quite right calling the booking detail can produce an error 500 on save or cancel. Once that shows up, all subsequent Front Desk operations will fail with Error 500.

In Joomla 3.1 the Joomla team removed the database error handling mechanism used since day 1.
This will require 1214 code changes to ABPro. Thanks for doing it to us again Team Joomla!
The error code is only hit if a database error occurs under Joomla 3.1, which should be never. However if a database or sql error is encountered the server can return an Error 500.
ABPro 3.0.2 will contain the new error handling.
If running 3.0.2 or higher, check your ABPro Error Log screen to see if and database errors are being reported.

Yet another thing that was found to cause 500 errors on one site was the header content type specifiers.
The site in question was getting "aborted: error parsing headers: duplicate header 'Content-Type'," errors.

The customer removed these lines from his getSlots.php and gad_ajax.php files and the 500 went away.

header('Content-Type: text/xml');
header("Cache-Control: no-cache, must-revalidate");
// A date in the past
header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");

From another user..
I had the component RSSeo installed, I have uninstalled it for now and everything worked!