Epson 3880 problem

I have Epson 3880 since this model was introduced. I am not using it a lot and still have original cartridges. The printer worked without any problem. Yesterday I was printing six copies of one print from a file created few months ago. The fifth print had a large round black spot on its margin (does not bother me), however the next print had a strike of black on the front edge of paper and it was darker. From now on all my prints are darker, it looks like almost transparent layer with black tint covering print. The effect is small however visible without any doubt, especially in skin areas.
I have checked all settings, ran Alignment and Clean Heads, nothing helped. It is not a matter of monitor, I am comparing prints from prepared files printed before, using same paper (Rag 308 loaded from back and tried also Epson Enhanced Matte), same profiles, and same computer MacBook Pro.
Is it possible that for example one of black cartridges was damaged and it is leaking, or may be cartridges are too old, can be anything else?

Have you ever changed the maintenance cartridge?

It could be full of ink, and needs replacing.

No, I didn't. The printer displays it is used in 54%, therefore I think it is still OK.