There was a paper jam in my hp P2055 laserjet printer. Of course, this is not a big news. However, after clearing it the printer is still complaining "jam in cartridge area" and gives advice how to fix the things, but the problem is, there is nothing extra visible any more inside the printer. How could I tell it to the printer, I mean, to make it start running again, which it refuses to do since we do not agree about the status of the things?

well one problem is this, the error messages sent to your screen are produced by some 'fault' in the printer (or sometimes computer) and in the printer there is z last of errors. The fault then 'chooses' the error nearest to its actual ability to detect the fault. Eg a microswitch at the bottom of the paper tray operated by no paper = paper out Obvious!
However a series of microswitches (one near the roller, one at the feed into the roller, one at the end of the paper out of the roller all detect some aspect of the paper passing through the machine (ie the top edge of the paper has passed by here!) and then finally perhaps (the bottom of the paper has passed by here!) But if the paper or some dust, or some ink, has prevented that switch (led) detecting the paper for some reason the only error message that it can give is ...yes you've guessed it 'paper jam). On my dell there is a duplexer unit (that means a plastic extra tray that turns the paper over to print on the other side). If that tray is even slightly out of alignment then it gives and error message yes you ve guessed right again (paper jam)!
I cannot therefore give you a simple answer like there is a piece of paper jamming, but it is certainly possible that there may be some other jam (roller or gear not running freely due to dirt, dust, age, no lubricant etc) that will lead to a paper jam message;
I think you will have to think a bit more laterally and see if anything is too loose or too tight, or dirty, and is the cartridge fully seated properly, is the cover of the machine fully closing properly etc etc.

Hope this helps

M