Hi Corinne,
What strange about this issue, is the Internal Server Error is not consistent sometimes it does appear suddenly and sometimes it does not, and if you get the error and refresh the page it will back to normal.
Try whitelisting this URI from the Mod_Security
https://www.cjchrservices.com/pro/
https://www.cjchrservices.com/cornerstone-endpoint
Let us know how it goes,
Cheers!