Hello Palal,
I think the root of the problem is that, when you click on "mange my bookings" in their website, it somehow is tied with the online check-in page which is not available until 48h before the departure. The "manage my booking" is different than the check-in papge but somehow, when I get the error, it links me to the check-in page. Glad to heard the plane is not overbooked. I reserved my seats in the plane too.
Inch'Allah