Hey Toby and thanks for getting in touch!
I just checked the site and was able to access both the calendar (/exhibitions) and a single event without a a 500 error, including using the link you provided. Were you perhaps able to resolve the issue on your own? Or maybe I’m missing something and you can help point me in the right direction.
I did, however, notice the following console error when viewing the site:
You have included the Google Maps API multiple times on this page. This may cause unexpected errors. (main.js:60)
I have seen other types of errors pop up as a results of this (e.g. the map not displaying on the site event post), so you’ll want to be sure to check where that extra instance of the Google Maps API is coming from–such as your theme or another plugin–and dequeue the script that is making that call.
Cheers!
Geoff