Lash eggs are not eggs. Rather, they are an egg-sized mass from the oviduct of a laying hen. And, they are a strange symptom of an illness that is the number one killer of laying hens. They are caused by salpingitis; an inflammation and infection of the oviduct.
Source: Countryside Daily, April 26, 2018. Link.
Salpingitis can be very hard to prevent. It is most common in birds that are two to three years old. Ensure laying hens are getting a healthy diet and free-range exercise time each day. Practicing good animal husbandry is helpful in preventing the spread of bacteria and viruses that result in salpingitis.