So far I had one theory. What if air inside the cartridge makes the print head fail, and cause the error. I thought that if a nozzle fires dry, it could, in theory, fry the resistor and make the cartridge "look different" to the printer, electrically. So I resorted to violence. I took a CL-41...
I've been refilling the Canon CL-40/41/50/51 series cartridges for at least two years now, and have had problems with them lately. When they first came around, they were by far the best inkjet cartridges with integrated print heads, at least what it comes to refilling. Not being able to reset...