Remi Lefevre wrote:

Errors encountered are "Underrun" errors (from bd).

Try elevating the arbiter priority for the FCC.

CPM clock-frequency is not defined in the DTS, neither filled by U-Boot,
but it doesn't seem to be needed, does it ?

Not for ethernet.

I'm a little confused, and find strange that the first UDP request completes
correctly, as well as latter ARP ones.

If it's bus contention (as underrun implies), then it may happen only 
under load.

-Scott