Welcome and my first response would be "look for another house." Yes it is serious and it could be poor design, building or materials withing the structure. Or worse, it could be that the entire house is settling on it's foundations. Or worse, there may be no foundations or again poorly made. There are way too may things that could be involved.
If your building inspector can't tell you specifically what is causing it then you need to find a competent building inspector. The step after that would be a local Structural Engineering firm.
If the house is going cheap there is a reason for it and any or all of the above thoughts will all be very expensive to repair.