The Interview Kit contains everything an interviewer needs to conduct a focused, purposeful interview and provide useful feedback afterwards. This includes instructions, the candidate's resume, and a scorecard. Creating thoughtful Interview Kits for each step of the process is the easiest way to align your team and improve you hiring process. When you schedule interviews, Greenhouse will automatically insert a link to the Interview Kit in the calendar item.
Additionally, this URL is how Greenhouse knows to match the interview scheduled in the external calendar to which 1) job 2) candidate 3) interview stage in Greenhouse. If any of these three criteria is 'wrong' or different, it will not be synced correctly between the external calendar and Greenhouse.
If you make any changes to this URL when you send out the interviewer invite and the URL becomes invalid, you will receive the error message as you have. Most common reason for this error is when the scheduler accidentally deletes the last digit of the URL or when they copy and paste the content, including the interview kit URL.