Hi Benjamin.
If you can show camera placement for this chair scan and some images (crops with only chair is enough), i probably can tell you the reason of this problems.
As i can see your raw mesh, for this moment probably 2-3 reasons:
Wrong acquiring (some surfaces have angle 45degree and more on images) or not have good coverage and overlaps.
Reflections or light spots on reflective surfaces.
Or surfaces with weak textures shot too far from them.
All this can be a reason that some areas on mesh have huge size polygons or holes.