We have resolved this issue and wanted to share what we learned.
One of my components that handles a USB device was working in 17.11 (x86_32, nova), but is causing problems when I attempt to run it in 18.05. The device is an FTDI-based USB-GPIO adapter (FT232H chip), along with a port of libftdi.
The USB hub setup seems to work, but the FTDI setup generates an error in the log "ftdi_usb_reset failed", occurring when libusb_control_transfer() fails during a USB reset.
We tracked the issue to a call to libusb_control_transfer() but the issue was not in libusb. Instead it came from us neglecting to set a pseudo-wall clock time as instructed in:
https://genode.org/documentation/release-notes/18.02#C_runtime
With that set properly our port of the FTDI GPIO library to Genode is working in 18.05.