You can get this message at any time. It means that, while Poser was allocating memory for the next bucket, it didn't find enough, even after reducing bucket size to the minimum (defaults to 32).
If you watch Poser rendering (good against insomnia), you'll see buckets getting smaller and smaller, then the dreaded message appears.

So either you reduce manually the bucket to something smaller (8, since you tried 16), or you simplify your scene (reducing texture size for instance, or layering), or you use the 3 GB Poser tweak (Posted here some months ago, sorry no link).