So I found what this error actually means for anyone that hits this with the same problem:
It's the asset name. It has to conform to Windows file naming standards otherwise you'll get this unhelpful error.
Of course the error message should be updated to "invalid asset name, please ensure that it conforms to windows file naming conventions" but that's a separate bug report to Microsoft.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…