We’ve now confirmed the cause of the blank ND/FMS issue in MSFS 2024:
We have also been made aware of a similar issue that is caused by updating the RJ’s navdata via the Navigraph Hub app. If the navdata is updated to the latest AIRAC cycle in the Navigraph Hub app, this can cause the ND and GNLU-910A FMS displays to remain blank after loading into the aircraft. Our current recommendation is to remove the latest AIRAC cycle in the Navigraph Hub app, and then reinstall the RJ Professional (MSFS 2024). This will allow you to use the RJ with the default AIRAC 2401 navdata that is included with the product. We are in contact with Navigraph to investigate this further.
Martyn - Just Flight