I've seen this message when the user was booked for previous events in the seminar. So it's not so much booking conflict (=booking for events happening at the same time) but sign-up workflow/condition.
These seminar settings control it: How many times the user can sign-up? and Restrict subsequent sign-ups to. If you de-select all options under Restrict subsequent sign-ups to the users will be able to book for any number of events.