I asked myself how they separate the usbinput and the 12v input why this works yesterday only when i plug the 12V too? Maybe the AFB26 is like a separator? And what happens when i change this to a 0 Ohm resistor? Has anyone replace this with 0 Ohm? Normaly the communication part has to work when only the usb is plugged in. But when only the 12V is plugged in the STM must be unpowerd.
As far as I know, the 5v USB circuit isn't related to the 12v power.
But if your ferrite AFB26 is damaged, it could be the reason why you have the failure.
You can replace it with a 0ohm resistor, at least for testing purpose.
I also have some spare ferrites. I can send you 1 or 2 from France for the cost of shipping, shouldn't be more than 1.5
Ferrites are used as a filter for bad quality power source.
When shorted, they can act somehow like a fuse, even if they aren't very good at this. They often damage the surrounding components or the copper tracks when melting.
I'd also check your USB power source. Maybe something is wrong with its voltage.