Firmware Updater: Flashing failed Board reset failed

Dear all,

I'd like to use the Firmware-Updater plugin on my Ender 3 with the SKR 1.3 Board.
I could follow the instructions, just had to add "chmod o+w ./usb", as I was doing all the operations with the root account.

Now the plugin can access /media/usb just fine, but it claims, that a Board reset wouldn't be possible (Flashing failed Board reset failed).

This is said to occur on older FW - but I'm running the newest Marlin! I'm also able to manually run the M997.

Any ideas what's goin wrong? Where could I dig further to find an explanation?

Thanks in advance,