Scheduling Poll - Recipients get email, but link doesnt work - does not appear on poll result page

I recently made Scheduling Poll available to all users in my Microsoft 365 org. One user is reporting a strange problem which has happened in multiple instances. He adds a group of people to the To: field, selects his proposed dates and times, and sends the email. 3 of the invitees received the email, but the links in the email did not work. Those 3 invitees did not appear on the poll result website on the right side. In another instance, one invitee did not receive the email and the name wasn't on the poll result page. Seems a bit inconsistent and I dont know if I have the full picture from the user. Just wondering if anyone else is seeing this. Thanks.

Update: More information. Invitee shared an error page he saw when clicking on the link stating "OwaUserHasNoMailboxAndNoLicenseAssignedException" with a 500 error. This is an external user to us. We've not had issues with other external users either.