Problem solved!
But I don't exactly know why...
I focused on changes in nVidia Profile Inspector antialiasing settings as per the manual for DX10SF plus antialiasing settings in DX10SF, let the DX10SF effects conversion tool make its job (it did that quite a number of times the past days) and voilĂ : PF40 McLeod and all the other CIRP stations show snow white blowing snow!
Yes, the problem was in connection with blowing snow effect, probably around snow drifts.
All ok now, I am now enjoying my Cessna and Piper ski planes out of CZST on Cambria Ice fields!
Thank you for all your friendly attempts to help my case!
Best regards
Ernst