Perhaps you have too many images and the thumbnail creation process (which is CPU intensive on the server) takes more time than allowed by your server setup. Reduce the count of images or the size of them and try again. Alternatively, if you have access to PHP's memory_limit, try setting that value to 64M, 128M or 160M.

In Joomla's global configuration, set error reporting to "maximum" and let me know. I need to see the actual PHP errors that cause the 500 server error message. It's also most likely you've exceeded your PHP's memory_limit.

As suspected, SIG is hitting your server's memory limit for PHP, primarily because there are too many images on the gallery in combination with something else running on that page that also eats up or requires additional memory from PHP, in comparison to the working URL.

Either request a higher memory_limit value for PHP (yours is 64M now) from your webhost or use less or smaller to resize images.

It may be because some other plugin is executed in the background as well, adding extra load to the server.

Either way you're using way too many images for a web gallery (purely from a frontend performance standpoint).