I would recommend trying to look at the added rasters to be sure none of them are corrupt or somehow different(ie undefined projection). If they are fine, then try to create a mosaic dataset in a file geodatabase and add the data that you just added. If that succeeds then I would call into create a Support incident as something is not correct. Usually that error would appear when there is an issue with the extent of the added rasters. In this case, it may be the extent is not correct, corrupt or the projection it is in will not fit in the coordinate system of the existing mosaic dataset. You can also try to add the new imagery a little at a time and see if you can determine which raster may be causing the issue.