Where does expandobjects.exe save its temporary files? (Parallel EnergyPlus simulations are crashing)

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ExpandObjects reads an in.idf and writes expanded.idf and expandedidf.err files in the same folder.

You might want to take a look at the \workflows\energyplus.py for an example or RunEPlus.bat to understand better.

In general, the note from energyplus.py

    # So...ExpandObjects is weird in E+.  It doesn't like running things in directories and accidentally symlinks
    # things into the current working directory even if you specify an output directory
    # We're just going to run it ourselves.  (Side note it could be a similar problem for EPMacro)

suggests to me that you might want to just run ExpandObjects prior to running EnergyPlus rather than using the -x approach. Perhaps, put the in.idf into a temporary folder.