Unfused means the toner is not fixed well on the paper. This hapened when the settings for paper in driver are not set correctly, when quality of paper is poor or if the fuser module is defective.
If I print a few pages of text in black color, text on every 2-3. page is "double printed". See photo below.
I replaced the black cartridge for a new one, but still the same problem.
Other colors (yellow, red, blue) is fine.
Thank for any advise