STM32F105 - USB OTG - BusFault at 0x1fff7a10
I found the problem.

It was in the file: usbd_desc.h

i missed to define the correct board.
So it tried to get the Device ID from a wrong address. -> BUS FAULT

How did i found the problem?

I looked at the stackpointer at 0x2000fedc and saw the address 0x8004196 this was the following c-code section: 

static void Get_SerialNum(void)
{
  uint32_t deviceserial0, deviceserial1, deviceserial2;

  deviceserial0 = *(uint32_t*)DEVICE_ID1;
  deviceserial1 = *(uint32_t*)DEVICE_ID2;
  deviceserial2 = *(uint32_t*)DEVICE_ID3;
  ...


Now i dived deeper and saw that DEVICE_ID1,2,3 pointed to a wrong address.
I fixed it and now it works!