@Flohack Ok, guess I'll just try it then and report back what happened... Thanks again for your help and advice!

@Flohack Sure! Could you please send me the link, I wasn't able to find it around here.

@Flohack So I tried the first command again, no change, same error :

ln: Die symbolische Verknüpfung '.halium-install-rootfs.thm7J/cache' konnte nicht angelegt werden: Die Datei existiert bereits

In case you don't know german, it just states that ln could not create a link .halium-install-rootfs.thm7J/cache because the file already exists.

@Flohack Nope, in the /data folder, there is a bunch of other folders(anr,app,app-lib,etc...) does this mean the first command was not working in the first place?

@Flohack 100% sure, big Downloading, do not turn off target sign.
BTW, I just realized that I didn't allow TWRP to write to my device. Could this be the Problem?

@Flohack And now I reached the next Problem:

• The first command appeared to be working, complaining about not being able to create a link in the end because the file already exists.

• Now for the bigger problem: In the last command, heimdall tries to download the PIT file but fails... When installing the TWRP before it worked without any Problem. I tried different cables and different USB Ports...

Any Ideas on how to find the PIT are welcome!

@Flohack Update 2: GOTCHA!
When I press the Vol. up key just after uploading the recovery and it automaticly updates, I can get into the recovery! So next thing for me is to try and install ubuntu touch on it!

@Flohack Soooo.. I tried installing the TWRP. The file is the same for both devices. Heimdall sent no errors, but I am not able to get into the recovery.... Neither Power + Vol. down nor Power + Home + Vol. down works. It always boots into the default recovery...

@Flohack Ok, guess I'll just try it then and report back what happened... Thanks again for your help and advice!

@Flohack I don't think i've got a second sim slot. Honestly IDK what the difference is, the only thing I know is that it's this model number. Just wanted to make sure I don't brick the system with a wron recovery. BTW, I read that in this thread that this smartphone runs android 5.1? My device never got an update over KitKat...? Strange...

@Flohack ok, thanks for your effort! While making extra sure I have the correct files and the correct device, it came to my attention that I have a GT-9301i and not a GT-9300i. So I guess I should not procced with the installation as it would probably brick my device?