Is your UR824 plugged in the exact same port as it was when you installed the driver? If you change ports, you have to re-install the driver, it is port specific.
It must be a direct (on computer) port, Hubs won’t work properly.
Perhaps a re-install of the driver might be of help anyways.
This is really an annoying bug, also on Mac. Installed the drivers again. Than it went ok for a while. Now its back. Only restart helps. Looks like Steinberg forgot this piece of hardware.
Unfortunately this annoying bug or whatever is back. Only restart brings the UR28M back. In Cubase its working. Only the dspMixFx_UR28M app brings up this msg. A fix would ben nice.
If you are using Cubase you do NOT use the dsp software, everything can be controlled from within Cubase itself, (which version are you using) so if you have the dspMIX running at the same time as Cubase, one of them will give this message
Years later, the issue is still there. Mac High Sierra, “Cannot use USB post; it is being used by another application”. Only restart can make it work again.