Distorted color on G3501 isn't fixed by head cleaning

[beneix]

I have a Canon G3501 that has been working without issue for a year. In the last week, however, it has started printing with distorted colors. I did a nozzle check and magenta seemed to be clogged. Cleaning and deep cleaning didn't help, so I removed the print head and cleaned it with an ethanol/distilled water mix. After that, the nozzle check comes out OK for the color swatches but the vertical bars to the left are green rather than black (apart from the top one). This is the output of the nozzle test:

The worst color problem is with red (R 255, G 0, B 0), which looks like this:

Since the nozzle check color swatches seem more or less OK, I am not sure what could be causing the problem. Any ideas most welcome!