I assume your SPOT images have RPC projections, or are in DIMAP format so that RPCs can be calculated?  How does the map information look prior to orthorectification?  Do the lat/lon values look good?  How about for the DEM?  If so, you may need to contact technical support directly for help. 

Dear mminari   Thank you for your reply.   I contacted SPOT technical support in relation to the RCP, and they indicated how I could open the image usign the DIMAP with no problems.   Prior to the orthorectification both the image and the DEM look good, fitting more or less the base map. The error that I mentioned only happened when I use a negative sign in the geoid offset (I calculated the geoid hieght as recommended by the ENVI help and it was = -10.945), as I tried without the negative sign and the error message didn't appear. However, in both cases the orthorectification runs but both products are much more shifted than the original image. How can this be?   Cheers

You should contact technical support.  They will probably need to see your data.