Bug with paint.net Thread

Hello,

I am making textures for 3d models using paint.net. The format used is dds.
Yesterday a texture was completely dark in game (normally it is a sand color). I've tried lots of things but I can't fix it.
I am sure of these things :
- it came from Paint.net
- I haven't modified paint.net (update or plugins) since approximately two month.
- only new texture are bugged : if I modify an old one, there is no problems.

Have you got an suggestion to resolve it ?

Thank you in advance...