Thanks guys.
Not yet tried to save as DXT5. DXT3 doesn't work for me.
Updated GIMP on all computers yesterday - especially on my Debian based Raspberry PI OS,
with which I work most of the time (installed the current version from the GIMP site after
deleting the one installed by Raspberry PI OS).
But maybe the issue is caused by my way of working and /or my available software:
creating files (xcf/psd) in GIMP/Linux and the dds in GIMP/W10 (didn't work in the previous
installed GIMP in Linux & doesn't work in an very old CS2, with Paint.net doesn't work neither).
So probably it's the combination Linux/Windows and Gimp/CS2/Paint.net
I'll give it another try tonight.
BTW: found that an old mod ("Historic TransAm" for GTR2) available on esport, which has the same issue
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So long.
Jeannomiller