Dear rick
Thank you very much for your answer. Using your suggestion I found that I have accidentally added some corrupted masks into my folder. But now I get lots of errors like this:
*+ WARNING: grid from dset full_mask.S211480 does not match that of dset zyxt
Almost all of the rows are identical, except for a few that differ. And the difference is very little :
full_mask.S175640 -3.312500 -3.312500
3.312500 196608 0
full_mask.S175741 -3.312500 -3.312500
3.310000 196608 0
Should I be worried about this WARNING ?