Anyone having a 'Lock-bit error' on a DM500S, try this:
(worked for me);

In Dreamup, after your box has been connected, powered up & acknowledged, click FLASH (not flash erase).

Select your image file.

A pop-up window will ask if you 'wish to carry out a flash-erase also' - Click 'NO'.

Your box will (should) now flash the image - you will notice it will automatically carry out a 'flash-erase' in the process anyway, then boot up the image you just flashed.

Hope that helps ;-)