Python in Visual Studio (PTVS): Debug not working

Question

I installed PTVS 2.1 on MS Visual Studio 2013 and then created a simple python project showing 'Hello World'. When I Start Without Debugging I see the shell and 'Hello World' message, everything is fine. But when I try Start Debug, nothing happens, no errors are reported, no shell comes up, no breakpoints get reached, nothing!

Any suggestion?

My platform and packages:

Win 8.1 64bit
VS 2013 Ultimate
Python 2.7.9 64bit
PTVS 2.1 64bit

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UPDATE:

I just found that in Visual Studio 2010, this problem does not exists.