Jochem
Since no-one has come in on this yet, I would say that I have recently had a few similar problems on my little R2400, always where one head loses a large percentage of nozzles for no sensible reason. In one particular instance, the printer was running absolutely fine until I changed a cartridge, and the subsequent purging routine actually caused blocked nozzles on a different colour. In my case, running two standard cleaning cycles about 1 hour apart seemed to fix things.
John