Hi ! After the battery slowly empties all my Victron devices stop to work (of course) But after the starting the mains charger all the devices starts correctly and connects via Bluetooth, except the MPPT 75/15 solar charger, have to disconnect/reconnect the +12V completely … This behavior is constant, every time.
A firmware fix should correct this. eg. reset the BT chips some second after 12V is restored.
With hope for a solution …
// Stefan
An easy cure
Don’t discharge the batteries down below 20% 
Interesting answer …. looks as a “work around" fix …
Just checked this with a not used 75/15 and can tell you, that it starts up correctly after the battery voltage is returing.
I decreased the voltage from 12V downwards, at about 4,6V the BT connection was dropped, then I increased the voltage rapidly back to 8V or so and it connnected again over BT.
Firmware is 1.64 btw. Only battery input terminal connected to a power supply.
So did you check if the voltage is really available on the battery terminals after “restart” of the battery?