OK, in this case you may need to download the individual binaries from here
https://github.com/particle-iot/device-os/releases/download/v1.5.0-rc.2/particle_device-os@1.5.0-rc.2+photon.binaries.zip
unzip them and apply them like this
(in Listening Mode)
particle flash --serial photon-bootloader@1.5.0-rc.2+lto.bin -v
(in DFU Mode)
particle flash --usb photon-system-part1@1.5.0-rc.2.bin -v
particle flash --usb photon-system-part2@1.5.0-rc.2.bin -v
particle flash --usb photon-tinker@1.5.0-rc.2.bin -v