Hi,
Sorry for being late to reply...
To answer your questions:
1. orthos are in JPEG2000
2. I discover that corrupted/damaged overviews has overlapping source rasters underneath!
3. I remade a new Mosaic Dataset with no overlapping rasters and it worked perfectly
What happened is the overlapping rasters in double were in 2 versions. A complete raster and a cut one! The Mosaic Dataset was building Overviews with the cut rasters... this explains why the overviews were not complete or damaged.
Attached are example of typical overlapping rasters, in double, showing that some source rasters were cut/corrupted.