Bug Fix for Build Errors in VS
After a lot of research and trying out I finally manged to fix the Bugs that occured for me (here) so I wanted to share what helped me.
I read this article and wanted to try it, but when I checked my VS settings the checkboxes were checked. So I tried to uncheck them and build but it still didn'T work. After that i rechecked them and then finally VS started to download some stuff. After that it finally worked.
TL;DR Check the boxed in Tools | Options | NuGet Package Manager -> General -> Package Restore. If they are already checked, try to uncheck them, rebuild and then check them again and build.
Hope this works for you too