Thank you Repairatrooper for your idea!
No, I haven't yet.
I'm studying now the "Color LaserJet Pro MFP M477 Repair Manual" - it seems pretty tricky to access the registration density sensor.
Is there a possibility to clean it without disassembly of ITB and the associated parts?
BTW, I'm using the original HP toner cartridges only.
- Marcin