I have wired up a VE.BUS BT Dongle to my Multiplus 12v 3K. The dongle is wired up to the main bus bar, not in the inverter, I have updated the software on both to the latest using the VE.BUS to USB and I have even bought a Victron RJ45 cable and still have the exact same issue.
It will all boot up and work but will cut out about every 30 seconds and reboot. You can see in the app through the dongle that it is as soon as it has got all the information (going from unknown to then give the inverting/battery details) as soon as it is showing the details it cuts out about 3 seconds later and goes through the same process again.
Don't know what else to try, might it be a faulty dongle as the Multiplus works fine without it?