I have a 1 year-old hen who has developed a similar problem. Since they began laying, I've always offered crushed oyster shell, free choice in a dish in their run. They eat a balanced organic layer feed. They also free-range 5-6 hours per day.
The rest of the flock lay perfect hard-shelled eggs. This hen, Peeps, used to lay perfectly-smooth hard shell eggs since she began laying at the end of October 2009. About a month ago her shells started getting increasingly irregular surfaced, warty, and covered with random granular "beads" and bead-lumps. Some beads flaked off, some were stuck to the shell. Then, last week she started laying these rubbery shell-less eggs: some from the roost at night, some in the nest box. Probably 5 or 6 of these eggs, so far. Maybe more, if the hens have eaten any that I didn't see. In the last week, she's only laid 1 hard-shell (albeit, warty) egg.
Since she has been laying these rubber eggs, she now "sits" on the ground sometimes when they are free-ranging. This is very uncharacteristic for her - she's a high-spirited, strong bird and pretty much in the middle of the pecking order. She looks very healthy, her poop is fine, she eats a lot, she eats the oyster shell. I've tried feeding her yogurt mixed with a teaspoon of cod liver oil and some raw oatmeal. She picks at it, but doesn't want to eat much of it.
Does anyone have any suggestions for me? I'm really scared that she has developed a defective shell gland. Is it possible to happen so quickly?