Hi Mitch,
thank you for your tips.
I've once again (checked the cartridge removal/installation instructions and) removed the cartridges in order to compare them and checked the contacts on their chips. No difference as far as I can tell. Still, I softly applied the eraser on the chip, just in case, but with no success regarding the cartridge error.
I've never used CWIS but I can't see how I could use it now - I don't know the printer's IP address and because of the cartridge error I cannot access the printer's system settings to look it up. Unless I can somehow access the printer's settings from my PC without knowing the printer's IP I'm stuck. (Isn't there a way how to assign the printer's IP manually in the same way I could do with PCs in my home local network?)
As for the new cartridge I haven't tried it yet because the new cartridges are quite expensive (not to mention the fact I was not interested in a new color cartridge in the first place, I rather wanted to get rid of the "cartridge almost empty" warning which was the only reason I tried the non-original cartridge. Otherwise I'd be OK with not having color toners because I hardly ever print in color (and the original Xerox cyan cartridge is not empty yet, anyway)).
Regards,
Radek