I guess if you connect in series you mean connecting the ground of sensor 1 to the output of sensor 2, and the ground of sensor 2 to the output of sensor 3. When this NPN NC proxy is active, the power supply voltage can be measured by measuring the output voltage. Because most of these proxies have inner tensile strength, they do. In this case, I would try to use a residential proxy and it should solve the problem. This will fix the connection that shows up as an error.