Hi, libvirt 3.2.0 introduce an annoying bug which is described here: https://bbs.archlinux.org/viewtopic.php?id=225121 When i remove the usb passtrough everything works, but this has nothing do to with the error message.
As IPFire is now running libvirt-5.6.0, I would propose closing this bug unless Jonatan indicates that this bug was not solved upstream and is still valid. Will wait for feedback.
Adolf, you can close bugs if you think they should be closed. The reporter can simply re-open if they think that this problem still isn't resolved.