Hello,
Had the same problem, or something that at least seemed to be, but got to the bottom of it on my machine. The symptom for me was that the newest Houdini version (18.0.287) didn't read obj files nor could it output obj-files that ZBrush could read. I hadn't had this problem in any previous Houdini version, opening the exact files that now proved troublesome.
On Odforce some people seemed to have found a different cause for similar symptoms - a faulty .env file.
https://forums.odforce.net/topic/28688-solvedhoudini-unable-to-read-obj-file/ [
forums.odforce.net]
This did not turn out to be the root of my problem. It instead turned out, that with Zone firewall I had chosen to block a weird suspicious repetitive outgoing connection by a program called qwavefront.exe. I didn't think of a link between Wavefront obj and this program when I blocked this suspicious behaviour, but it turns out there is one.
It turns out this suspicious program needs to get to connect to the internet every time Houdini reads or exports (a working) obj-file.
I hadn't noticed it before, but it seems, at least now, that the same is true on Houdini 17, although I don't remember such firewall alerts from when I was using that.
Anyway, I got the problem fixed by allowing qwavefront.exe to the internet, but why it needs that connection when the same doesn't seem to be true when reading or writing obj-files with ZBrush, nor with a few other 3D-applications I've used (SpeedTree, Cinema 4D), still remains a mystery. Overall the new Houdini version aside from it's undeniable awesomeness, has a problem of seeming to want to connect to the internet a lot more than previous versions, for different reasons (continuous license verification, I think, is one that didn't seem to be as constant a need in previous versions, but there seem to be other processes as well). I quite hated Autodesk Maya trial version for having constant processes that had such outgoing and incoming needs. I do hope Houdini isn't following in the footsteps of similar programs in these practices.