Thank you! I guess this worked. But it wasn't the only thing that prevented Fedora from booting but also a misconfigured X Server...
Before this all happend I had tried to install the nvidia drivers together with bumblebee and it was necessary to reconfigure the X server until I could boot normally again.
For those who might have the same problem I will shortly outline how I was able to solve it. At first I followed rknichols suggestion and replaced the "default.target" - to point to a valid target (see post before how to do it).
Thereby it is to mention that since I ran the commands from a live system I had to adjust the paths a little bit.
At first I mounted the partition Fedora is installed to, using Nautilus (I don't know how to do the decryption from the shell). Than I looked up the path it was mounted to - per default this is "/run/media/<partition name>", in my case "/run/media/Fedora" if I remember it right.
Than I set the working directory in a shell instance to this path by doing
# cd /run/media/Fedora
Than I could execute the commands given by rknichols but had to remove the leading Slashes when manipulating the link (run/media/Fedora/etc/systemd/system/default.target), since this isn't an absolute path in the live system.
# rm -i etc/systemd/system/default.target
rm: remove symbolic link `etc/systemd/system/default.target`? y
# ln -s /lib/systemd/system/multi-user.target etc/systemd/system/default.target
After having done this I rebooted in text mode by pressing 'e' at grubs system selection screen and adding " 3" (without quotation marks) to the first line.
There the system did not fully start but after a plenty of tries I noticed that I was to able to switch to a console by pressing Ctrl + Alt + F2 (or any other F-Key except of F1) where I could login successfully and after executing 'startx' and rebooting the whole thing ran fine again.