Power over Ethernet identification error?
Find the solution :

After a LOT of tests it turns out the problem was the Earth connection of the shield of the Ethernet cable.
In our design the shield of the Ethernet connector was connected to the ground through a ferrite bead of 220 Ohm at 100MHz, so we replaced it with a capacitor of 2.2nF/1kV (like the one for decoupling fly-back converters). 

We also had an "Earth loop" problem, sometimes we need to have an Ethernet connection AND a power supply, but you can't have two different Earth on a system. You can found more information in this document (in French but schematics are comprehensible).

Hope this helps !