I have just watched this myself and when it fails the following is displayed:
action index=95704 type=HII
error:[80001801H] The action handler input is invalid.
From digging around looks like I need a patch - OSD-5242390.2 - will contact support.
Patches have been applied and it's no different! Anyone seen this behaviour? Support are being slow at the moment.
You need to apply the MCP's (at least Core, OSD and SD) with the date of April. Lots of things are fixed in that update. Be careful that also a new boot.wim will be installed. You might wanna remove this from the installation or restore your SP2 boot.wim after.
Oddly this seems to be fixed by turning ON the Auto Detect option, whereas previously it needed to be OFF or builds failed. Weird.