I was attempting to provision a (used) DL380 G8, purchased without disks/OS. USB wouldn't work for anything (e.g. no keyboard, couldn't select USB drive for OS install, etc.)
The machine was plugged into our KVM Switch (Trendnet 8-port), although I was attempting to use a locally connected keyboard (the KVM keyboard didn't work either).
Disconnecting the KVM entirely (using a different monitor, etc.) seemed to allow the USB subsystem to work. OS is now installing.