Horizontal black lines on CP1215

[JoelStienlet]

Dear all,

I've a problem with my laser printer CP 1215: When turning the printer on in the morning, or after sitting idle for some time, it prints pages full of black lines of the first pages, then the number of black lines diminishes, and after 10 pages (more or less) the prints become perfect again.
Please note that:
- The prints are absolutely perfect between the black lines.
- The pages affected by the black lines are somewhat dirty on the backside, some toner deposits on the back.
- I join a photo of several consecutive pages. You can note that, where a picture should have been printed (the black rectangle), there is some darker black blob that seams to smear out, even on the following pages. But the white spaces keep completely white, which means, the "dark blob" doesn't affect the white spaces.
- The toner is (was) new.

I've found what I believe to be the exact same problem on a "Laserjet Pro CM1415", in a thread from 03-01-2013 on this forum, question without a solution: https://h30434.www3.hp.com/t5/Printers-Archive-Read-Only/Laserjet-Pro-CM1415-prints-a-page-full-of-H... 
You can see what has been tried, but did not work (changing the toner cartridge, updating the firmware). But I'm pretty confident that in the meantime a solution to this problem has been found.

Thank you very much for your help!