Error Message with Flashing Lights (power, internet, paper feed)

Ah, yes. The HP 6X mech; these are sorta troublesome and kind of sensitive. This usually occurs when the carts are moving due to one of 3 issues:

• Carts are hitting something in the printer, or there's an impact stopping it -- likely an impact, especially if you heard a "click".
• Paper jam somewhere deep in the mechanism, or in the jam door.
• Motor failure (rare, happens)

The first thing I checked when I had my C4780s (Yes, I went through a few of them because I ran them hard and they always had issues with the motherboard or WiFi module which kills them. These used the HP 60, the one you have uses the HP 64 or 65 cart? The one you have is the same DNA with a different cart, smaller chassis) is if the carts are loose, this will do it.
NOTE: this is not often the case as it requires a hard push and clicks when it's in. If it was loose, you'd know.

As such, it's likely a deep paper jam in the mech tripping a sensor somewhere; get a good flashlight and check all over. You also want to check the jam door as well; PULL SLOWLY on these. If you can't find anything, take it apart and look for an internal jam.

If you do not see one, it's either a bad sensor or a bad motor. In those cases, it's usually BER. I hope you bought reman carts (though this sometimes causes errors as well; but 95% of the time it's the printer). The only time I buy OEM ink for used inkjets is when I know it's good, not an unknown (Ex: High-end wide format Epson or Canon I got from a photo studio for next to nothing over a higher-than-average PC with a user-swappable waste ink pad). The "downside" is they can have more wear, but they're also proven and aren't shelf queens with tons of nozzle clog issues. If I HAVE TO buy OEM due to something like the "black cart" serialized Epson chips or someone activated HP+ (if I haven't dumped it because of that "feature" locking me out of my choice to run 3rd party ink and solved that problem by taking it out of the population of units which can be reused); others are just too new for reman carts to be common or just aren't as common, like the wide format Epsons (Ex: SureColor P series, Stylus Photo Pro; both are not as commonly cloned due to the high yield ink, in some cases the inks are 80mL and the market is REALLY particular due to what they get used for)... I have a no-exceptions policy: The printer must be cleaned and flushed before I put actual ink in it with a flush cart. Is it specific? Sure, but if I have an Epson with 9 80mL OEM CARTRIDGES I'm not chancing it in terms of waste; especially on the ink DRM machines since the cleaning fluid is cheap.

I don't think this is a waste ink error, these just make a mess on your desk when it's full and stop absorbing waste ink. Only Canon and Epson have WIC counters which shut them down that I know of reliably stopping it.