Add to calendar options giving error 200 and 3001

Hello!

I made a webinar and sent the links to the add to calendar option through our newsletter (we've done this before and it worked fine). This time only I can add the meeting to the calendar since I'm logged into the account that created the webinar, but anyone else who got the email could not.

Error codes are 200 and 3001. I thought it was because we're using HubSpot and they might be adding a UTM, but that wasn't the issue at all. It's just because they aren't logged into the account that made the webinar that's why they can't add to calendar.

Anyone help, please? Thank you!