I checked the print head connections, but without any succes.
Thank you everyone for qualified comments.
In conclusion, its a new print head (rather expensive) or a new printer.
I use my Canon PIXMA 3000 under Linux Mint (Isadora). It worked perfect until recently, when pictures looked pixelated to some degree. When running the nozzle check, the pattern looked systematic as shown below (vertical lines were perfect and omitted here)
_________________________________...