Vantec UGT-PC345 USB 3.0 adapter fails in first x16 PCIe Gen3 slot, HP Z240 Tower workstation

[MRFSYS]

this EN LABS item has 2 x extension cables BUILT-IN:

Enlabs MB20P22U310IN 2 Port Internal USB 3.0 Motherboard 20-Pin Header to 2x USB 3.0 A 10-Inch Female Adapter Cable,USB 20pin to 2 x USB 3.0 Splitter Cable -Black

https://www.newegg.com/black-en-lab-10-inch-25cm-usb-cable/p/181-01CY-00002?Description=USB%203.0%20...

[DGroves]

the group that created the pci-e spec "REQUIRES" that all newer revisions to the original 1.0 spec be completely backwards compatible with it

 as such pci-e rev 5 devices will work with all previous versions 

all major venders of pci-e based products do extensive testing of consumer products to confirm compliance so the odds of your product not being pci-3 compliant are just about zero as such the problem you are having with the card are most likely due to other things such as two cards not being compatible with each other, or a improper bios setting or a bad/corrupted OS or a corrupted card driver (in your case usb 3 drivers are part of win 8 or newer)

i would set the system to bios defaults, and remove all other cards except for video, and if still having a issue try a bootable usb key of a OS and see if the card is detected/works correctly

the above is not to say that specialized non retail consumer low production products might not be tested for older/newer pci-e revisions, (such cards do exist)  but the odds are that such a product would still work it would be a very rare card that was not backwards compatible hardware wise with the PCI-e spec 

[MRFSYS]

On the theory that the problem may be unique to the Z240's primary x16 PCIe 3.0 (x16 electrical) expansion slot,

I'll try the Vantec card in a different PCIe 3.0 expansion slot (time permitting).

A TP-Link TL-NG421 2.5GbE adapter is currently installed and running AOK

in the x1 PCIe 3.0 expansion slot, which is currently DISABLED,

so I can remove it for testing the latter theory.

Similarly, a TP-Link TX401 10GbE adapter is currently installed and running AOK

in the secondary x16 PCIe 3.0 (x4 electrical) expansion slot, which is currently ENABLED

and running at 2.5GbE.

See WARNING above, repeating:

if it is not being used for a graphics card, only cards certified as After Market Options for this platform are supported

MANY THANKS!

[MRFSYS]

SUCCESS!

This was the TIP I needed:

"if it is not being used for a graphics card,

only cards certified as After Market Options for this platform are supported"

I removed the TP-Link TL-NG421 from the x1 PCIe 3.0 expansion slot,

and installed the Vantec UGT-PC345 in that same x1 expansion slot.

Windows 10 installed the driver automatically,

and the driver shows up normally now in Device Manager.

I'm testing the Vantec card with a 64GB SanDisk USB 3.0 thumb drive,

and there are no apparent errors, thus far.

The problem was evidently in restrictions that are unique

to the primary x16 PCIe 3.0 (x16 electrical) expansion slot,

normally used for video cards.

MANY THANKS to everyone for all your help today.

[Prométhée]

You're welcome @MRFSYS 

Glad this info put you on the right track.
but all credit goes to you..
well done, well done!