VS 2015 Community Edition - Test explorer window is blank

I'm having a problem with Visual Studio 2015 Community. The test explorer window is empty, no tests are showing up, only the buttons for grouping and parallel execution are there.

I'm using NUnit, but also tried the build in MSTest Framework. The NUnit Test Adapter is installed and all tests are discovered and executed.

I've tried the following things to solve the problem, but nothing has worked so far:

• Restarting Visual Studio
• Uninstalling/Reinstalling the Test Adapter
• Reset Visual Studio settings (http://ift.tt/2aekobc, http://ift.tt/2aqRacf)
• Reset window layout