Your theory is proving correct: I put some ink into the (previously unused) LK cartridge, and the printer then started making a noise for a minute or so and then displayed error message 1404 which apparently indicates a vacuum failure. When the printer is turned on, it pulls ink from the cartridges but fails because the seals aren't sound.
Perhaps these cartridges have simply started to fail after 2.5 years. The chip still communicates with the printer, but the cartridge isn't delivering the ink.