There was no problem before the switch to VS 2010 and framework 4.0. So after some googling here is the solution :

• Edit your nunit.exe.config (should be in your Program directory next to nunit.exe)
• under <configuration> add :

<startup>
  <supportedRuntime version="v4.0.30319" />
</startup>

• and under <runtime> add:

<loadFromRemoteSources enabled="true" />

Sources :

• http://stackoverflow.com/questions/3542904/nunit-2-5-7-requires-explicit-debug-attach-under-vs2010
• http://stackoverflow.com/questions/930438/nunit-isnt-running-visual-studio-2010-code

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