Good evening Dan and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
We are sorry to hear about the error 500 appearing on your website.
I would love to help you with this topic.
As a first troubleshooting step, could you please provide us with your complete system information in a private reply using the instructions found in the following link?
https://theeventscalendar.com/knowledgebase/sharing-sys-info/
Typically error 500 messages are caused by a corrupted .htaccess file
It is possible that something went wrong with that file at the same time as you updated.
The first thing I would try is to re-save your permalinks after any update
Secondly, if that still does not work, you might want to try to temporarily use the default WordPress .htaccess file and see how that goes: https://codex.wordpress.org/htaccess
Finally, to answer your question, the latest version of the Events Calendar Pro is 4.1.2 (even though our free plugin is at 4.1.3)
Best regards,
Geoff B.