I quite don't understand what's going on, but the current state of the Symfony packages is broken.
The simplest example is Silex:
@fabpot this is because the dependencies between the different Symfony component are very strict: they force using self.version.
But the initial install selects the stable 2.1.0 tag for some components but chooses 2.1.x-dev for HttpKernel. This breaks things later as symfony/security depends on both HttpKernel and HttpFoundation which have been installed with different versions.
@stof: So, changing all self.version to 2.1.* would fix the issue?
@fabpot Yes that should do the job.
Only in the require but not in the replace of the main package though I'd say.