Thank you everyone for your time and advice.
It just doesn’t make sense - these artifacts simply shouldn’t be there with Blender’s default settings - never seen such ugly effects on any scene rendered from Blender. Could it be I changed some default render setting by mistake? Perhaps that’s the most reasonable explanation…
The issue here were indeed those distractive circles/waves visible in the background of the render but they surely can’t have been caused by compression, since I’ve tried multiple file formats, including *.png, *.tif as well as pure *.bmp (no compression at all on that one, afaik). Finally, there was no image imported whatsoever, only pure RGB colors with some specularity applied onto the objects.
Thanks for the link. However, turning the BIAS setting on/off or changing the SBIAS/LBIAS setting doesn’t seem to be affecting the render in any way whatsoever in my case. And neither did ClipStart/End as advised in the article, even with only 1 frame.
So far, this has been actually the only working solution, although I’m far from being sure it’s the “right” one. It does remove the ugly lines between transitions from brighter areas to darker areas (more/less light) but one can notice the “dithering” effect or diffusion of the pixels in the image slightly. And those artifacts only disappear after the last pass while they’re still visible throughout the whole rendering process, which is quite odd.
Now I’m wondering what the default Blender value for the “Dither” option is because I doubt it is actually “2” (as long as I didn’t reset it by mistake).
So, this method might just work but I still don’t think it explains how this can possibly be happening in such a plain scene and with default settings - I assume that’s not regular behavior in Blender.
Tomorrow, I’m gonna do a fresh Blender install on sister’s computer and see what happens there…
Thanks again for your help and support.