Good evening Mark and welcome back!
Thank you for reaching out to us.
We are sorry to hear about the 500 error message creeping up on you. Yikes!
I would love to help you with this topic.
If you have not already done so, I would recommend going through the following: http://www.wpbeginner.com/wp-tutorials/how-to-fix-the-internal-server-error-in-wordpress/
Secondly, can you tell me if this happens when you are updating normal events or recurring events ?
Finally, can you tell me if the issue persist if you Try temporarily reverting back to a default WordPress theme such as twenty-sixteen ?
Hang in there!
Geoff B.