MSTest fails: Failed to find configuration section 'log4net' in the application's .config file

This is my setup:

/
solution.sln
Core/
    Core.csproj
web/
    web.csproj
    Web.config
Test/
    Test.csproj
    App.config
build/
    _PublishedWebsites/web/etc...
    test.dll
    test.dll.config
    Core.dll
    log4net.dll

I wrote Unit Tests in the Test project that test the functionality of the Core project. All of the projects use log4net, all of them are configured using the respective App.config or Web.config files using

[assembly: log4net.Config.XmlConfigurator(Watch = true)]

in their AssemblyInfo.cs.

The projects are built using NAnt into a build directory.

When I run the Unit Tests from Visual Studio (2010 Professional), everything works fine.

But when I try to execute MSTest directly

MSTest.exe /resultsfile:TestResults.trx /noisolation /testcontainer:build\test.dll

All test cases fail with the following error:

log4net:ERROR Failed to find configuration section 'log4net' in the application's .config file

This confuses me, because the test.dll.config file most definitely contains the log4net configuration section.

The folder created by MSTest also contains the config file:

/flopes_MyMachine 2015-10-08 08_48_59/Out/
    Core.dll
    test.dll
    test.dll.config
    log4net.dll

It appears to me that maybe MSTest is using a wrong configuration file or doesnt find the configuration file of the test.dll? Does that even make sense? Is there a way to tell MSTest which config file to use?

What is going on here?