Small update:
i have flashed the new bmc and bios over bmc before…
After reflashing the bios manually (not through bmc), it works… Finally 
Reboots are working now, boot times are okay now either.
It’s still slow, because of configuration of pci and whatever, but it’s not comparable to before.
It boots now in 30 seconds or so, instead of 2-3 minutes.
I think the bmc flasher stucked somewhat in the flashing process? Because for me that looked like on every boot the bios gets flashed over and over again, that’s why it took so long and reboots didn’t worked…
I don’t know, i think that the bmc flasher is broken simply.
Cheers 