vogtländer - I am sorry I didn't see this earlier. That message will happen for various reasons. The best place to start is by typing "rtl_433" at the command line to see if it will connect to the USB stick. If that works the second thing to do is to make sure the iobroker user has permissions to access the port of the USB stick as well. There are extensive instructions on the rtl_433 site to set the permissions on the ports for non root users.
Let me know what happens at the command line