Thank you very much Martin!
The issue was only a memory limit problem. The number of results displayed exceeds over 300 items, which caused the process to run out of memory. I have increased the memory limit by adding the following line to the wp-config.php file in your root directory:
ini_set('memory_limit', '512M');
It should be okay now. Nevertheless I suggest lowering the number of initial result here (I recommend max 50), and using the load more results feature, so it will load more results once the user scrolls to the bottom of the list. That will greatly improve the performance and the server load.