But as far as I know the Optiplex 760 has a PCI-E 2.0 slot, which is fully backwards compatible.

• Hewlett Packard 435508-b21 - Network Adapter - Plug-in Card, Quad (4-port) Ethernet, Fast Ethernet, Gigabit Ethernet
• PCI Express 1.0a, Fits in x4 slot
  

PCI-E 1.0a is not compatible with PCI-E 2.1 bus and line code.

This is why newer cards dont work in old systems like the Precision 490 690 etc with PCI-E 1.0 bus.

This is a known issue since 2009.

https://www.intel.com/content/dam/doc/guide/pci-express3-phy-implementation-considerations-idf2009-presentation.pdf

8b/10b Line code is not compatible with PCI-E 2.1 and 3.0 and up 128b/130b line code and bus speed.

PCIe 1.0a data rate: 2.5 GT/s

PCIe 2.X data rate: 5 GT/s

Doubled the data rate/ bandwidth from Gen 1 to Gen 2

The card is too slow and does not understand the line code for PHY.

The PC in which I tested the card has PCI-E 3.0... But there are no problems... Therefore I think the Optiplex is the problem...