Hi –
Jumped on Avid this morning. Firstly I can replicate the error by playing the sequence backwards. It doesn’t do it on scrubbing but hitting J then waiting a few frames then get the error.
I turned off all the audio layers and the problem went away – although the video didn’t play back great backwards (I’m not sure if it is meant to). It is only DV in this case (film offline).
I also tried copying the Avid MediaFiles dir to a lacie FW800 drive and unmounting the XSAN – same error.
The computer is this:

and has this Fibre card:
Fibre Channel Domain 0:
Vendor: LSILogic
Product: LSI7204EP
Revision: Firmware 1.3.14.0
Bus: PCI
Slot: Slot-2
Initiator Identifier: 126
Node World Wide Name: 20:00:00:06:2B:11:15:A0
Port World Wide Name: 10:00:00:06:2B:11:15:A0
Address Identifier: 01:06:00
Description: Port 0
Speed: Automatic (4 Gigabit)
Topology: Automatic (Point-to-Point)
Status: Link Established
Cable Type: Fiber Optic
with these cards in the system

The other video card listed is a Blackmagic Decklink HD Extreme 2. This is our slowest machine – I can replicate the problem with our Mac Pro 3,1 8 core also (same Fibre and Blackmagic card).
Thanks for your help!