Ahh. My mistake.

No, it was just the material itself. And by removing the parentheses from the material names, I was able to fix that problem.

On the other hand, I've just conducted a quick test by creating an .obj file named sphere(geo).obj and importing it into Poser. And sure enough, the object is greyed-out in the Materials panel (see above).

Poser doesn't care for parentheses, it seems. There might be other time-bombs lurking in your keyboard, so be mindful of those naming conventions, everyone.