I have a very powerful machine too, but the failure to reload textures error occurs for me now and then after I've had a complex scene open for awhile and run some renders.  I believe the experts have reported that Poser fails to release memory as you go along and run through processes like rendering.  I've found that for memory intensive scenes, it is best to save before rendering (always a good idea anyway).  If you get the texture reload error, you can just exit Poser (dumping the memory load), re-enter Poser and reload the scene.  That always works for me.  Another trick I've used to hang in there a little longer in a given session is to set the display to outline or wireframe BEFORE you render (so it doesn't have to load the textures again for the working display when it returns to Preview mode).