The Windows Boot configuration data file does not contain a valid OS Entry

When starting OS deployment from ConfigMgr you get the following error message: The Windows Boot configuration data file does not contain a valid OS Entry. This because Windows Deployment Services (WDS) has been manually configured at some point. The best course of action at this point is to reset the installation of WDS by reinstalling the PXE Service Point and WDS.

What also can be the cause is that deployment is started on an unknown system, and Enable Unknown computer support isn't checked at the Distribution Point settings. Just select this option and deploy a task sequence to All Unknown Computers collection too. After that your headache will be gone :-)

Source: Gerry Hampson Device Management (comments)