Until now i used to select a transparent image and copy paste it in another image (let's say just a background color). 2 days now, i cant do it any more. I changed everything to default, with no luck. Is there a setting to do it? I read everything in web how to fill a transparent background with color but nothing works.
If you can paint on opaque areas but not on transparent areas, make sure that the layer is not "alpha-locked". The alpha lock is the checkerboard icon in the
Lock: line at the top of the Layers list. When it is set (this is sometimes hard to see), you cannot change the opacity of the pixels, so you cannot paint on transparent pixels.
If you cannot paint on the opaque areas, then the "layer" is possibly pixel-locked (paintbrush icon in the
Lock: line), but in that case you should have a warning (if you are using the Gimp console dialog, check it).
PS: note that the locks apply to the layer (and not the image) and that they are saved with the image, so even if you reset Gimp to factory settings, they will be set when you load the image.