Then the new owner has probably already assigned your ProfiLux 3, which is possible by using the combination of myGHL-ID and a freshly generated security key during the myGHL-assign process.
If there are still issues left, please create a support ticket where you can provide the P3 serialnumber (if still known) and your myGHL-account:
https://www.aquariumcomputer.com/sup...ets-enquiries/