I got that problem just a few minutes ago with an old scene I loaded up.

In my case, it was because I had disconnected Poser from the particular Runtime in which the textures were located.

As soon as I added a Poser Library for that Runtime again, it was able to find the files once more, without me having to tell it where to look.

Hope this helps!

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)