On our custom system with AT91SAM9263 - DP83848C connected through RMII, we see following networking problems:
When connecting network before start-up, we get a valid IP-address.
When connecting after complete startup, most of the time we don't get a correct IP-adres, only an "eth0-avah" -address.
When reconnecting the network (disconnecting longer than downtime), most of the time we don't get an IP-address either.
"ifplugd"function seems to work correct.
When trying to shutdown ethernet using "ifdown eth0", most of the time we don't see a kernel message. When no kernel-message is given, the "ifup eth0"-command gives no good result.
"/etc/init.d/networking restart" gives no good result either.
Timings referred to XTAL on DP83848C are all ok.
Suggestion of connecting RX_ER to GND gives no good result.

Does anyone know a solution for this problem?
Thanks for looking into it and helping is out.

>>Does anyone know a solution for this problem?

Sounds like a hardware or configuration problem.

Thoroughly check the schematic, and confirm all the parts, and no-fits, are correctly placed on the board, and of the correct values.

Use a scope/analyzer to look at the signals on the board, and cable, and confirm there is nothing obviously abhorrent.

Write some standalone test code (ie runs on the board without Linux/uBoot/whatever) that can bring up the interface and test/exercise basic functionality, and register settings/status.