To summarize for other people having the same issue which is beeing stuck in fastbood mode:
You’re stuck in fastboot mode if your screen shows something like this:
and the 4 options (start / restart bootloader / recovery mode / power off) will only bring you back to this screen after rebooting your phone.
Then you should do as followed:
First try a soft reset by pulling the battery, sim cards and memory stick out and in again. If this is not working, then:
Download the newest
A link is provided further down
Connect your phone to your computer and
Extract the .zip and click shift + right mouse
you will see the option to start a power shell mode: do it
Check if your device is recogniced by your PC by typing:
fastboot devices
if you get a response you might proceed.
You might find this Code near the top:
If this is the case go for
Then it should return something like this:
Setting current slot to 'b' OKAY [ 0.016s]
Finished. Total time: 0.031s
Then type fastboot reboot
which should give you a response similar to
Rebooting OKAY [ 0.016s]
Finished. Total time: 0.016s
If this is not working try:
fastboot --set-active=b
fastboot reboot
If this is still not working you might have a problem with different versions of fastboot.
Then you should follow this guide:
Redo
If it is still not working I dont have any more clues 
Thanks for this fast and great help to k4y0z, AnotherElk and Johannes!