Hi,
I was currently looking into your issue, it seems to be related to some modification done for Substance Painter 2018 in the command lines.
We found from where it comes from, and we will try to fix it as soon as possible but I can't give you an ETA at this moment.
In the meantime, you can use the "Use existing spp" option (this way you shouldn't get the error).
Cheers,
Ludovic