How can i install the Windows Feature .net 3.5 on Windows 8? (Error 0x800f0906)

I just cant get it to install.

This is what ive done already:

• Followed this guide and activated the group policy

• downloaded an 8.1 Image and added the /sources/sxs folder as an additional source

• tried to use the command Dism /online /enable-feature /featurename:NetFx3 /All /Source:D:\sources\sxs /LimitAccess

but nothing worked. I need this feature urgently, and i have no idea what else i can do except completely wipe the PC, which i obviously would rather not.

EDIT: By the way, when trying to install it offline through the dism command, i get this error:

[===========================66.0%======                    ]

Fehler: 0x800f081f

Die Quelldateien wurden nicht gefunden. (Source file not found)
Geben Sie mit der Option "Quelle" den Ort der Dateien an, die zum Wiederherstellen des Features erforderlich sind. Weitere Informationen zum Angeben eines Quellorts finden Sie unter "http://go.microsoft.com/fwlink/?LinkId=243077".

Die DISM-Protokolldatei befindet sich unter "C:\windows\Logs\DISM\dism.log".

Its basically telling me to use the /source flag to specify the file, because it could not be found. Well, i did exactly that (what the guide told me). Id love to give the command a more specific path instead of the general /source/sxs folder, but i dont know what file its looking for.

Concerning error 0x800f081f: I already made sure to check permissions, its the right image (Windows 8.1 german, just like my OS), and im running CMD as admin.