Hi Niels,
Thanks so much for reaching out!
I’m sorry to hear that you are having an issue with your orders. Unfortunately, as per our Support Policy, we cannot log in to your site, but we will do our best to help you here.
Is the example you provided the only order missing from the Attendees List? If there are others, can you see a pattern for the reason why these orders are not registering as Attendees? It is possible that some payment gateways do not add to the Attendee List. The example that you provided me with purchased with a direct bank transfer. If you set the order to Completed, do you see any changes?
I see that there are other orders marked as On Hold that do show up on your list, however the one missing from the list is marked as Guest. Does altering this field change anything?
Here is a forum thread where a customer created some custom code to fix the issue, you are welcome to try out a similar solution: https://theeventscalendar.com/support/forums/topic/attendees-missing/
Please let me know how it goes and if you have any other questions along the way!
Thanks,
Jaime