The communication between the powerlines seems to be causing this issue. You already know that the Slingbox is working properly since you do not have this error with a hardwired connection. I recommend to confirm that the Ethernet cable that you are using is not too long (more than 7ft long). If the issue persists you can try to use this troubleshooting steps for the Slinglinks, they are a Sling Media product but they are powerlines too.
Hope this helps