Ran into this exact issue before. No real resolution was achieved, since I could much more simply swap cards into working machines. Dell X520 cards are cheaper on ebay for good reason.
AFAIK, their firmware (dell variant of the firmware and rom [separate, afaik]) instantiates at SMBUS addresses that are normally occupied on regular ATX computers.Sometimes, you're lucky and there is no collision. Sometimes, you are not so lucky. It is a bus with addresses that must be non-colliding, like the old IRQ# you used to have to set for devices/peripherals and traps. However, many SMBUS devices don't really seem to have a way to let the user change their address or r/w commands easily.
You can either tape over the SMBUS pins on the card or somehow get the card to accept a more stock Intel firmware and rom. Intel did not release a stock image into the wild, and most rom+firmware dumps just won't burn using the regular tools. I had over 25 bookmarks saved from various resources w.r.t. doing this, and ended up writing my own tool to help assist this.
In the end, it's just easier to put the dell card where it works*, and remember why pulled Dell X520s are so cheap. Someone probably has a better solution. A really, really bad trick to use is the hotplugging PCIe, which works across reboots. Obviously, it will still fail on a cold boot, which is less viable for a router or any critical device.
*I have an Asrock AB350 ITX that works with the Dell X520 card, so I just use it there. I also have at least two boards that don't work with it.