from arch wiki -> https://wiki.archlinux.org/index.php/bumblebee#Installation
ERROR: ld.so: object 'libdlfaker.so' from LD_PRELOAD cannot be preloaded: ignored
You probably want to start a 32-bit application with bumblebee on a 64-bit system. See the "For 32-bit..." section in #Installation. If the problem persists or if it is a 64-bit application, try using the primus bridge.
I already tried that.
I guess I'll give up (for now). In the end it's not really important, since I still get ~120 FPS with my internal gpu. Just would have been cool to get that working :p
Switching the distro is not really an option for me - except for Arch. I was both on Ubuntu and Mint for about a year each, and despite I actually liked the user experience they gave me, I want to avoid Ubuntu and its derivates for privacy and personal reasons. I tried out Debian, but I didn't like it as much as Arch / Manjaro. I would have gone to Arch, but that seemed to be too complicated for me still. I'm sure I'll go there one day, enjoying Manjaro as a good step towards Arch. :-)
Anyway guys, thank you for your help. I really appreciate it.
If someone has any other idea (sometime) I will be happy to tried it out!
Cheers