We had a nasty little gremlin slip into our Online Bookings settings that we've just patched up! This issue would affect accounts with their "Minimum advance time required for bookings" set to Tomorrow.
With this problem, we would offer slots that were available tomorrow and that were under 24 hours away. However, when your client would make it to the confirmation screen, they would receive an error saying the booking needed to be more than 24 hours away.
This has now been fixed, so that when we say tomorrow, we mean tomorrow (regardless of the time when the booking is made today)!