Error 500 by press sending button

Hello.

Probably, its because of conflict with some other plugin or actual theme.
Please try to deactivate one by one all your active plugins at the WordPress > Plugins menu and make testing after each plugin deactivation.
If its not help, please deactivate all your plugins and active the default WordPress theme. And then retest it again.
We just need to find the reason of this issue.

Kind Regards.

These steps did not resolve the issue. My wordpress URL and site URL are different could it have something to do with that? And if so can I change the ajax-path to be called?

Hello.
In general it’s really weird error, at your website:
PHP message: PHP Fatal error: Class ‘WP_Locale’ not found in /home/domainname/domains/domainname.nl/public_html/test/wp-content/plugins/booking/core/wpbc-translation.php on line 85

1) In WordPress “Class WP_Locale” defined at inside of the file:
../wp-includes/class-wp-locale.php

2) the line, which is generate error is called from the file:
../wp-settings.php on line 461:
do_action( 'plugins_loaded' );

And at the top of this file (line 155) WordPress must to load this file
require_once ABSPATH . WPINC . '/class-wp-locale.php';
where is defined that Class ‘WP_Locale’

3) wp-settings.php file called from the ../wp-admin/admin-ajax.php
So here is evrything is fine.

For me it’s seems that something wrong in your WordPress installation with loading of class-wp-locale.php file in point #2

I do not sure, why system was not loaded that file with Class ‘WP_Locale’, whcih is standard WordPress way of file loading.

Did you make some customization that was override this workflow ?

Kind Regards.