Good evening Falayn and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
We are sorry to hear about the fatal error message you have been getting. Yikes!
I would love to help you with this topic.
As it turns out, this is a legitimate bug for some of our users. We apologize for the inconvenience this is causing.
A fix is underway. Unfortunately, I cannot commit to a release date at this point. But stay tuned, you will be contacted when the fix is released.
In the meantime, we have a workaround available.
Try the following: https://theeventscalendar.com/support/forums/topic/http-error-500-when-trying-to-save-attendee-info/#post-1163856
Let me know how that goes.
Best regards,
Geoff B.