Because this is happening to multiple chickens at once, that might be a toxin. Have you seen any mold in their feed container? I am not a vet, and cannot tell you exactly what is happening, but I would try to rule out several things. Mareks is always a possibility, but it usually doesn’t happen...
You could call Poison Control (US) about the creeping cucumbers, which are not good for animals to eat. There are some flushes such as epsom salts or giving some drug store activated charcoal, but since it has been 5 days, I’m not sure if it is too late to work. I would try to get them all...