Are you using Windows Server 2003 R2 (or not R2)? Do you have SP2 installed?
You might be running out of server resources because of the differences in the way the older version allocates resources. You could try disabling the option ROM for any adapters where you are not using the additional options. That will free up some resources.
It looks like you have an almost identical problem to mine: http://communities.intel.com/message/116311#116311
Mine is on Windows 2008 SP2 64-bit and I have also noted that it works fine on Windows 2008 R2 and VMware 4.1.
If you found a solution, please post.
For fun(?) I installed Windows 2003 R2 64-bit on my Dell R710 with the X520-T2 and get the same result as you. I get a yellow icon in device manager and a "This device cannot start (code 10)" message also. I'll confirm that my hardware also works with Windows 2008 R2 and VMware 4.1. Hoping for a solution.
This issue is probably not a driver issue, but I know I should never say never. Therefore, if you are willing, I have an older package, version 15.4.1, with 64-bit Windows* drivers that you can try. This is the first package that included support for the X520-T2, but has older drivers than those you might have used for testing.
The driver package is at http://downloadcenter.intel.com/Detail_Desc.aspx?agr=Y&DwnldID=19305. The description does not mention Windows* Server 2003, but those drivers are in the package too.
Let me know if the older drivers make any difference.
I am also wondering if there is some difference with the way the older Windows versions are allocating resources to the PCIe slots. You might be able to make some BIOS setting changes that could help. There are BIOS settings that affect resource allocation. Dell would have the best advice on potential changes to BIOS Settings.