Offizieller Kommentar
@Klem > First, be sure that the XDJ-RX is selected as the device under the Settings page, DJ Device option.
Some other possible answers (these say CDJ, but also apply to other products):
• Some models of Android devices do not support the MTP mode. For information on whether or not your Android device supports the MTP mode and how to set it, see the Android device’s operating instructions.
• There are some Android devices on which operation in the MTP mode differs when an SD memory card is inserted and when no SD memory card is inserted, depending on the specifications of the MTP mode of the Android device you are using. With such Android devices, it may be possible to connect the device to the CDJ-2000nexus after removing (or inserting) the SD memory card.
• There are some Android devices on which connections in the MTP mode may only be usable with applications exclusively for Macintosh or Windows, depending on the specifications of the MTP mode of the Android device you are using. Such Android devices cannot be connected to the CDJ-2000nexus using a USB cable.
• There are some Android devices on which the “MTP in use” message is displayed continuously in the foreground while connected by MTP, depending on the specifications of the MTP mode of the Android device you are using. With such Android devices, the CDJ-2000nexus cannot be connected to or operated from rekordbox (iOS/Android), since the application cannot be operated on the screen while connected in the MTP mode.