Hi!
I suspect that some data might have gotten corrupted when saving and it throws an error.
Try the following steps first:
1. Make a back-up copy of your database first, just in case
2. Try to delete all search instances and create a new one instead
3. Go to all of the submenu pages (compatibility options, caching options etc..) and save the options once again
If this was indeed the suspected problem, it should disappear.
If not, then make sure to enable debug mode in your wp-config.php file, maybe there is an error message preceeding the error 500 page, which could be helpful.
If none of these change anything, then I will need temporary FTP and back-end access to debug through the plugin code to find what is causing the problem.