I was still using Xcode 7.X, so I updated it to 8.2.1 by downloading the latest version from the App Store.
Now Xcode seems unable to load certificates. When I open my project (Mac app), in the MyApp.Xcodeproj viewer, in the Signing segment, there is the loader (spinner), and it won't go away.
When preparing the package for App Store distribution the validation process fails as it says that sandboxing is not configured. However I am sure it is, as this very app is in the Store.
EDIT I've completely removed Xcode (inclusing com.apple.dt.** files) and reinstalled it, the problem persists, but only with this particular app. Creating a new app does load the signing section.
The solution was unexpected: out of options, I simply created a new project (with a different name). The idea was to copy all code and working settings to start fresh.
As I begun the process the "new" app was working, so I renamed the project as my actual app and then I opened the old project to copy some xcode settings. With my surprise, now the Signing section was working again also in the old app.
So I figured that I could delete the "new" app. However, after deleting the new app, the old one stopped working again (spinner in signing section), this time also mentioning that it was unable to locate certain files that were in the new app folder. I recovered from the bin the new app, and finished the transition of all settings. Now everything is working again.
My assumption (not verified) is that some config files deep in the OS were conflicting with signing setting for this particular project (as identified by its name), so when I renamed the new app with the same name of the broken project I must have overwritten such broken setting.