What could cause those speckles on my printer's output?

A while ago, I have bought a used HP CP5225dn printer. I knew that the device was not 100% OK, but the price was very good, and a complete set of nearly new toner cartridges was included, so I decided to buy despite the following drawback: When I print something, there are often speckles in the output like the ones shown in the image below. This happens independently of the sort of paper (glossy, normal), of the size (e.g. A4 or A3, but see below) and of whether I print in color or black / white. I always adjust the printing options according to the sort of paper I actually use, and I always u

Question by Binarus on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.