If the spoiler deploys at the correct speed and retracts at the correct speed the warning light suggests there is a limit switch that is not working right. It may be bad but it could just be a marginal electrical connection. The switch is unable to recognize the spoiler is in this case fully deployed.
I no longer have my Boxster reference resources to refer to for the locations of the switches, but consider looking for the limit switches of the spoiler and carefully disconnect each one from the wiring harness connector then just as carefully reconnect it making darn sure the connection is 100%.
Start with the switch that is supposed to recognize the spoiler is deployed and at its most upright position.