It sounds like you might be missing a required PHP extension, check the Install_Requirements
page of the installation guide to make sure you have everything you need. You should also be able to look in your PHP log file (which might be your webserver log file, depending on your configuration) to see more information about the error 500.
If that doesn't help, try enabling debugging and enable SiT's own debug log file (see Debug_Logging
) which might help you find out more information about the problem. Post back here if you need more help.