Thanks for all that great detective work, @Lieke! Seems we should update the instructions.
One thing I also wanted to mention is that in general you should really only need Validate if you're manually editing XML. Tools like http://opendatakit.org/xiframe/ or XLSForm offline automatically run Validate after converting your forms (if you want to be more technical about it, it's actually that the tools are all built on top of pyxform which itself runs Validate).
If you do need to manually edit XML, it would be great to know what you're doing that requires it so that we can look into getting some of that functionality into XLSForm.