We just noticed that the previous and next navigation on the single events are not linking correctly. All events link to the very first event added (one from early 2017). On your demo-page the links go to the previous and the next event.
What can be the problem?
This is working on the demo, so I would recommend running through these Standard Troubleshooting Steps to confirm that this is working on your install with just the parent theme, and theme plugins. From there, you can start adding back in your plugins and customization to identify where the break is occurring.