I’m not sure if this problem is connected specifically with log4net or this is some problem with VS.
Everytime I’m trying, I have the same result. But let’s start from the beginning.
I’m creating a simply console application (.net 4) and using nuget to install the log4net library.
Now, when I want to add some code from it, intellisense works ok:

But when I try to compile it, I have an error

This is the only one assembly I found to this moment, I have this problem with. I have absolutly no idea what (and why) is going on. I have checked with two other solutions and problem is the same. I have not checked manually downloaded version yet.
Most likely there is an underlying dependency that is missing. One very common reason is the project is using the Client Profile instead of the full .NET 4.0. (Not sure about log4Net, but I run into this a lot when including third party libraries, esp. those with ties to the web). If I am correct, simply going into properties and changing the framework version to full .NET 4.0 will solve the issue.