I have the same desktop board, and read in the Product Guide that there are two Front Panel Headers.
See page 50 and 51 of "DP55WG_ProductGuide02_English.pdf" which you can download here:
The Alternate Front Panel Power LED Header (page 51) is specifically for a three-pin power LED cable. So may be this is what you need.
If not, the same Product Manual states on page 75 that the POST Code 5A means "Resetting SATA bus and all devices", so it could be related to one of the devices that are connected to SATA connectors.