Not enough calcium, or more likely, just a fluke.
I have a hen who lays two eggs for a day and then skips a day. The second egg of the day is always shelless.
We have two hens that lay shell-less eggs. I'm getting nervous that one of these days our hens are going to get smart and figure out that if they brake the shells of the other ones, they'll find more prizes inside.
I'm going to pick up some grit and try it out, soon, but mine all have access to layer rations, oyster shell, and partial free-range.
Could be a vitamin issue. Try giving the hens a little cod liver oil. I dip small pieces of bread in the oil to get them to eat it. It will help them absorb the calcium and get them into sync.....
I have a Salmon Favorelle that's about 14 months old that has recently started doing the same thing - without normal eggs in between. Sometimes I get a very thin and soft, broken shell with the egg material. I feed a pelleted feed, grit, oyster shell, and a little cracked corn, plus scraps from the house every few days (mostly veggies). Everything else is normal, she still sits on her empty nest, and the shelless eggs are never in her nest, always out on the floor...
I too would welcome any ideas!!