C++/CLI: the debugger can't hit any breakpoint

I'm extending a pure C++ solution using C++/CLI one with VS2008: it's a dll that gets invoked by our main program. For debugging purposes, in the same solution I have a separate project that links to the public interface of the DLL. Nice and easy, right? Yes, until I added the /CLR switch to the compiler command line; from that moment on, I was unable to debug the DLL, since VS couldn't reach the breakpoints I set, showing them as hollow circles with the yellow alert sign superimposed and informing me that no source code could be found. The precise error was:

"Breakpoint will not currently be hit. There is no source code available for the current location"

To make a (quite) long story short, the problem was in the settings of the debugger: you have to set the debugger in mixed mode in the project of the exe calling the dll.