RSVP Error occured while processing your response

Hello everyone, I would like to ask the issue regarding this one? Sometimes when I submitted the form I redirected to this page instead of the confirmation page. The thing is I didn't do anything with the code, does anyone know the issue of this? Thanks

also I have emailed them see screenshot below for response, however

    

The link they provided is not working anymore, I hope you could help me thank you.