I see whats going on.
So in the admin settings if you type 1.50 then on front end it wil show as 1,50 and total calculations work.
Kindly use 15.00 on admin side for 15,00. Event though it shows with a dot in admin side, on front-end it works correctly.
I know its a minor inconvenience, please try this as it works on front-end and we will work on resolving this issue for admin side in the mean time 🙂
Completely understandable with the sensitive information Miriam 🙂
Thank you very much for kindly sharing that information! And ALSO I very much appreciate your patience and flexibility in working with us on solving this issue!!!
Ok So the term query is very taxing on your website.
Are you able to install query monitor plugin https://wordpress.org/plugins/query-monitor/ and send me a screenshot of the database queries on staging site with event type queries? like the attached.

here is the code
https://codeshare.io/Od9xVr — please replace this into file includes/class-intergration-webhooks.php inside rsvp addon folder.
Hello there, thank you for explaining further. I just released an update to RSVP addon. Give us some time to work on this and share you the code to support the additional fields for the webhook string 🙂
I went ahead and refunded you for the order for the 3 addons.
Hello Yannick, thank you very much for taking the time to explain what you are looking for. From what you said, it seems a booking plugin would best suite you over a event calendar that offer booking as a secondary functionality.
We wish you best in discovering a solution that best meet your expectations! Kind Regards Ashan!
Hello Mark other order has also been refunded and subscription cancelled. Thank you for recognizing the mix match of your expectations vs what we could offer. We wish you find a solution that best fit your expectations!
Since uninstalling and reinstalling the plugin, all information is showing correctly

I have un-installed and reinstalled the plugin, it appears to be working okay but I am getting the error messages as shown in the attached image
And here is a third video showing the data missing again on the same events that were updated earlier. Please help me figure out what is going on.
https://www.loom.com/share/736a104378f14c4ab16290caa95d0c36