WCF in Azure Load Balanced servers

Hi,

We are currently testing our website in Azure.

There are two web servers with one load balancer.

We configured the website and the WCF in both server.

However, the error as the image below appears once a while.

https://ibb.co/k9YkKw

Is there anything we need to set to make it work in the load balancer environment?

It works all the time in our testing server without load balancer.

Thanks.