Hello,
I am having a similar problem. I added the code snippet as shown on the page:
https://theeventscalendar.com/knowledgebase/change-default-categories-view/ to my functions.php file. The main calendar works fine, but when trying to access a category view, I get a server error 500.
All other extensions were turned off and I reverted back to the parent theme, which is Divi, but still have the same issue.
I am using Event Calendar Pro, version 4.0.5
Any information would be greatly appreciated.