Anyone struggled with the same problem:
According the specs the F3 line card should support FEX 2000 (in our case: NX2248PQ and NX2238TP)
While configuring an interface with "SWITCH MODE FEX-FABRIC", the system complains with:
DC1(config-if)# switchport mode fex-fabric
ERROR: Ethernet4/10: Configuration does not match the port capability.
Seems like the only interface on the F3 linecard that is able to support FEX fabric, is the BREAK-OUTed interface (4x 10GE) on a F3 interface.
Kind regards, Wim