I created a simple scene with a planet in space. I texture-painted a plane as the background and that worked fine, just a black plane with lots of colored glowing dots.In the first picture renders it worked just fine displaying white stars at the colored dots, but after I rendert an animation the background suddenly turned greyish/orange affter the first frame and from now on it doesnt changes back.
Did you save your hand-painted image texture externally? When you first paint a texture in Blender, it's only saved in cache. If you change frames, close/reopen the project, or move stuff around, you risk losing that work.