You have an error while sending to the server. Check out the Network panel to see what was the request, and what got returned by the server that caused the jQuery error.
Image creation is done on the server, and jQuery sends to it the information needed to assemble the image. Apparently, jQuery sent the request, but the server did not comply. When this happens, the error message you need is the one returned by the server.
For example, a 404 error would indicate that you did not install the necessary HighCharts server-side components.
A PHP error such as function not existing might instead indicate that you not have some necessary software installed (typically the GD2 module, libjpeg, and/or ImageMagick).
Finally the 500 Internal Server Error indicates that there is another error behind that one, and it can be found in the Web server's error log. It could be for example a .htaccess problem in an ancillary plugin directory.
The Web server error log on shared hosting can usually be checked/cleared/downloaded from your hosting provider's control panel, or you can check it directly from the site configuration file path setups if you have SSH access (check ErrorLog entries); if all else fails, it can be usually found in /var/log/apache... on Unix/Linux systems.
You can also check out the Web request that caused the error, and inspect it visually to assess whether, for example, you do not have such a path (that you know of). This would then indicate that there's an installation problem.
Finally, you can check that path (/dev/php/export), and modify the source of its index file to track down an error. This is sort of a last ditch effort if no other debugging is possible:
<?php
// This is the index.php file that receives the POST
print "<pre>"; print_r($_POST); die(); // DEBUG-ONE
// The above line crashes the request, but should avoid the 500 Error
// and show the POST information.
...some code from the original file...
die("<pre>".date("Y-m-d H:i:s") . " No 500 Error so far");
// After commenting the DEBUG-ONE, processing goes on and
// should arrive here. If it doesn't, obviously the error
// is between there and here.
// etc.
