It’s been a long known fact that installing the .NET Framework 2.0/3.5 Feature on Windows Server 2012 and Windows Server 2012 R2, doesn’t work without the sources. You can get around this issue by mounting the install sources and navigating to them, when installing the .NET Framework.
Another way to do it, is by using the following command: Dism /online /enable-feature /featurename:NetFx3 /All /Source:<source location>sxs /LimitAccess
After some digging, I found out that you can’t install .NET Framework 3.5 if you have additional Language Packs installed, which was the case with me.
The only way to solve this, seems to be, to remove the additional Language Packs via lpksetup and then installing .NET Framework. You can install the Language Packs again afterward.
Sometimes, only: Install-WindowsFeature –name NET-Framework-Core –source <drive>:sourcessxs, seems to work with Windows Server 2012 R2.
It also seems that Windows Security Update: KB2966827 seems to be one of the causes, that .Net Framework can’t install. Removed this update and then installing the framework seems to work.