The following was logged in smsdpmon.log during content validation:
CSMSDPMonitoring::ReportPackageState failed; 0x80004005
CSMSDPMonitoring::ReportStatusMessage failed; 0x80004005
This occurred because the distribution point was using a self-signed certificate and the management point is in https mode.
The fix was to create a create and issue a custom client certificate template for distribution points as specified here.
System Center Configuration Manager Notes 