Ron, the drop detector calibration is ok, this Error Code 524288 doesn't come up on the printer LCD panel, it's only visible through the event logs on the service support pages of the webserver.
[attachment=18825:524288.jpg]
Since the scanning system is ok I think it's time to replace the printheads as stated in the service manual at page 3.16: "If the problem persists after using valid paper, cleaning the printheads, and keeping the window closed, there may be a Failure in the scanning system needing reparation or the printheads, although clean, may not work and need to be replaced."
However, the printheads are all out of warranty, but checking through the front LCD panel of the printer the status of all printheads is still OK. I will get new printheads at the beginning of January.