I just had the exact same problem, after several successful builds. After a reboot that error showed up.
I think the Source is already shown in log.htm file.
The Source is Ultimate Vista 5600 version same as previous I tested version.
And I do nothing add, delete, edit, change anything on it.
I do not use WAIK source.
I found wmfiltr.sys in my %windir%\sytem32\drivers directory. I found the attached .inf file on the Web. I copied wmfiltr.sys to the same directory as the .inf file, right-clicked on the .inf file and selected "Install" ... and the problem was fixed.
wimfltr.inf.zip 1.15KB 331 downloads