"Printer Jam in Cartridge Area" HP Laserjet 401dn, but there is no paper jam

These false paper jam errors almost always crop up when I am printing duplex, every few pages.

I have RTFM's numerous web pages, but I still don't have the answer. The only "fix" seems to be removing the print cartridge and pushing it back in hard. I'm worried that my approach will break something long-term.

Not much we can do to help with a physical printer problem. You say the only fix is removing the cartridge so use the user manual to find cartridge replacement. It will tell you how to remove and reinstall the cartridge correctly.

Also try removing and reinstalling the sheet feeder. Take a flashlight and see if you can actually see a piece of stuck paper inside. If that doesn't fix it, I would suspect one of the electronic boards in the printer has gone bad which many times are either not user replaceable, no longer manufactured, or so costly that it doesn't make sense to replace. At least this is what I have found to be true in HP deskjets. Even if the board is user replaceable, you will probably have a tough time finding one for a 7 year old printer plus you have to narrow it down to which electronic component it is.

Here are the guides for that device. https://support.hp.com/us-en/produc...jet-pro-400-printer-m401-series/model/5096260

i found this to add to 'glasskuter' advice.

and this

sorry not much of a help.
best of luck, Steve ..