Hi Manu,
Can you confirm that you have the latest version (0.7.3) of the IMC installed, and that the location of the IMC application is correctly set within ReefMaster?
If that is the case, and you still have the same issue, could you also please try to export exactly the same mosaic using the KML super-overlay or MBTiles option and tell me whether or not that works, and if you get a usable mosaic image at the end of the process. Check this by looking at MBTiles files within RM (check the option "add to workspace" during the mosaic export), or by opening the KML super-overlay file in Google Earth.
thanks,
Matt