virtualbox.org

VB is 5.1.14 r112924 linux.amd64 with Extension Pack 5.1.14 r112924, no Guest Additions.
Host is Xubuntu 16.04.1 64-bit, memory 16GB. Also seen on hosts Slackware-Current 64-bit and Windows 10.
Guests are Linux liveCDs such as PartedMagic 2017_01_08 (32-bit), Porteus 3.2.2 Mate (64-bit), Ubuntu Mate 16.04.1 (64-bit) and so on, memory 2 GB.
When attaching a USB key via Devices > USB an error "Failed to create a proxy device for the USB device. (Error: VERR_PDM_NO_USB_PORTS) pops up. This happens when the guest is selected but not started yet as well as when the guest is running.

If it's Ubuntu 17.04 and similar ultra-recent guests then the first thing I'd do is choose the Ubuntu-64bits VM template. A 2.6 kernel template is certainly not correct.

Hopefully you've also read section 3.10.2 of the user manual, specifically the bit about joining your host user name to the vboxusers group?

Resolved via https://www.virtualbox.org/ticket/15050.

Thanks for reporting back.