Was modelling a lamp yesterday,rendered it (one on the left), then saved and closed Blender. I then came back to the file today, added some objects to the scene, then rendered it again (right), somehow the lighting has gone wrong. Afaik no settings were changed, can anyone help how to revert to the one on the left?
Those look like artifacts created by the denoiser. Use more samples to render and lower values for denoiser.
Or you might want to try the Intel Open Image Denoiser instead of the built in one.
Try to uncheck Multiple Importance at light source: