VMs won't start on Fedora 23 [closed]

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Hello, I'm new to Linux, former Windows user. I use Fedora 23 and installed Oracle VirtualBox on it. Boxes was already pre-installed. I want to use VirtualBox, and I can set up the VMs, but they won't start. I installed it following several tutorials, such as this or this. Reboots included. I always get the error Kernel driver not installed (rc=-1908). It says that I have to run /sbin/rcvboxdrv setup. I ran the command, the output is:

Stopping VirtualBox kernel modules                         [  OK  ]
Uninstalling old VirtualBox DKMS kernel modules            [  OK  ]
Trying to register the VirtualBox kernel modules using DKMSError! echo
Your kernel headers for kernel 4.2.6-301.fc23.x86_64 cannot be found at
/lib/modules/4.2.6-301.fc23.x86_64/build or /lib/modules/4.2.6-301.fc23.x86_64/source.
                                                           [FAILED]
  (Failed, trying without DKMS)
Recompiling VirtualBox kernel modules                      [FAILED]
  (Look at /var/log/vbox-install.log to find out what went wrong)

So I looked at /var/log/vbox-install.log, and the output is:

Uninstalling modules from DKMS
  removing old DKMS module vboxhost version  5.0.12

------------------------------
Deleting module version: 5.0.12
completely from the DKMS tree.
------------------------------
Done.
Attempting to install using DKMS

Creating symlink /var/lib/dkms/vboxhost/5.0.12/source ->
                 /usr/src/vboxhost-5.0.12

DKMS: add completed.
Failed to install using DKMS, attempting to install without
Makefile:194: *** Error: KERN_DIR does not point to a directory.

Now that I looked it up for this post it has changed (??). It was something like
Specifiy KERN_DIR=<directory> and execute Make again
But I know that the environment variable is set because I executed
KERN_DIR=/usr/src/kernels/uname -r
export KERN_DIR
AND
export KERN_DIR=/usr/src/kernels/uname -r
several times for me & root user.
I searched dozens of forums & tried it with Secure Boot mode on and off.
The only thing is, in this thread, it says that the outputs of the following commands should match:

rpm -qa kernel | sort -V | tail -n 1 

uname -rm

They don't. Output of the first command is:
kernel-4.2.8-300.fc23.x86_64
Output of the second is:
4.2.6-301.fc23.x86_64 x86_64
It works neither on Boxes nor on VirtualBox, I tried VMware too, but I don't like it.

PLEASE HELP!!! (and excuse my (maybe) bad english, I'm 17 years old and from germany, I did my best.)