Most probably you manually increased the availability of the room for a problematic period and forgot about it, so there is an overlap.
If this is a problem, then contact us directly and write us detailed situation such as:
Problematic Reservation ID
Affected dates that are in overbooking
When is the check-in date
Try using Messanger chat on the bottom left part of the screen so that you get the best support for these matters.
Also please make sure that if check-in is less then 48h from now try to offer to the guest alternative accommodation and we will found the reason fo the error in the meantime.