Fault on first start of the day

Hi everyone, i`m experiencing a failure on the first engine starting attempt (rotax 912 uls on a RV-12) on cold engine.

This is what happens at first start.

-Disconnect battery from solar battery charger and maintainer
-Turn proppeler for oil check
-Turn EFIS on
-Master on (electric fuel pump on)
-Ignition both
-Choke
-Ignition key.... the prop just move a couple of inches folowed by a rattling noise (in the firewall) and the electric fuel pump fuse blows.

30 seconds later.

-Second start attempt (with electric fuel pump off, fuse blown)
-Ignition both
-Ignition Key
-SUCCESSFUL IGNITION the engine turns on and runs as normal.

After the engine starts you can restart it without trouble as many times as you want thru the day with the electric fuel pump on

The battery is allways full charged (anyway i perform a load test and it is in very good condition) the plane flyes twice a week.
I do not know if the rattling noise is from the starter solenoid, or from the master solenoid.

Does anyone have an idea what`s going on before i remove the starter?