I just looked into it, and it has to do with the em_bookings database table...
The person_id, doesn't match a wordpress user ID. So if you already have bookings, you will see an error message for each one of the bookings that don't a user id associated with it.
This can be fixed by doing these steps:
To fix the current bookings already registered:
1. Create a booking only user with the role of "Subscriber",
2. Go to the users table within your wordpress database and see what the user id is for the new user account you just created.
3. Copy that user id and then go to the em_bookings table and update the person_id column for each registrant where the person_id doesn't equal a registered wordpress user. Update it with the id that you just copied.
4. That should remove each error message that you were getting on the front end as well as within the admin panel.
Now to fix the Events Manager Settings so that it doesn't happen again:
1. Login to the wp-admin and go to Events - Settings - Click on the Bookings tab.
2. In General Options, I have Allow Guest Bookings set to yes,
3. Now, in the No-User Booking mode I have "Enable No-User Booking mode" set to Yes
4. Allow Bookings with registered emails set to no
5. Assign bookings to ... I have this set to the same user I setup during step 1 above.
6. Click Save Changes (all)
So what I'm doing here is allowing visitors to register without signing up for an account. Then I'm assigning all of the bookings under the subscriber account that I setup so that the database has an actual user id associated to it for each booking.
I hope that this helps!