WDS "Failed to process client unattend variables"

I'm on Windows Server 2012R2. I've installed a Standalone WDS Server and created two unattend files with WSIM. WDSUnattend works perfectly, ImageUnattend doesn't work at all. The error "Failed to process client unattend variables" can be found in the setup log. The ImageUnattend file works fine if used instead of the WDSUnattend file or is merged with it. It simply is never loaded. I have tried just about everything and I've run out of ideas, It's also not very encouraging that apparently about 20 other people had this problem and no one got a solution.

I have tried setting the ImageUnattend file via GUI and WDSUTIL, same result every time.

Any advice is appreciated.