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Did you (or any repairman for that matter) set the toner/developer ratio differently than usual?
If so, changing that back to something more normal or its former value, could help.
Were you using remanufactured or refilled or chip-changed toner cartridge or cartridges?
If so, that can contribute to this error also.
The cartridge (if it is the cartridge) that is most likely to be at fault is the Magenta.
Clean the TCR Sensor window on the underside of the Imaging Unit, if dirty.
Reinstall the imaging unit. (from one web page... I think what it means is, Re-Seat Imaging Unit.)