I thought it was a disk IO error, so I tried to open the file alone and it worked.
So I deleted the entire folder and copied it again from the portable disk, but the error still happens.
I have two different folders with different bias frames, one with bias acquired on 10 august and the other one acquired on 22 september.
When in ImageIntegration I load first the 10.08 folder, the error ever happens on the same file of the 22.09 folder.
If I invert and load first the 22.09 folder files, the error ever happens on the same file of the 10.08 folder.
So, maybe is there a file number limit that can be integrated at the same time?