tried to flash the firmware using dfu-util but 2 issues occured:
Device not available
After putting the core into DFU-mode (press+hold mode button, reset unit, release mode after RGB LED flashes yellow) I changed the driver by using zadig to WinUSB successfully. dfu-util didn’t show it up, I tried to reinstall, checked if I made everything according the manuals.
Solution: There seems to be an issue if the spark core is connected to a USB 3.0 port. Change to a USB 2.0 port resolved the issue immediately!
dfu-util quits with error message "Error: Last page at 0x08004fff is not writeable"
This (might) be a typo or whatever.