A “500 — Internal Server Error” is a very ambiguous and hard-to-track error.
Troubleshooting a 500 Error usually requires to have access to the server to make modifications in the code in order to find the origin of the problem, and I cannot ask you to provide administrative access to your website to do this as this would create liability.
Please disable the plugin for now, I will try to reproduce the issue and include a patch during the next update that hopefully will fix the problem. If you are willing to try the development version of the code (code that includes multiple bug fixes since the latest release) feel free to download a copy from here [1] I will mark this as resolved and move it into our internal issue tracker, my project manager will assign a priority to this case and notify me when I need to work on the fix.
[1] https://github.com/cixtor/sucuri-wordpress-plugin