Signing up for an event returns 500 error
So after much digging, I found it was due to Civi trying and failing to send an PDF receipt.

After disabling this in Administer > System Settings > Misc (civicrm/admin/setting/misc?reset=1), the error resolved, the 500 Internal Server Error disappeared and the emails started sending with the email receipt instead of the PDF.

Attach PDF copy to receipts: No