I just deleted everything in the dev account and my keychain related to the game I was producing. Then I went through step by step, making sure there was only one certificate and one mobile provision, etc. So for betofantasioso, if you are still having trouble (3 months later), here's some things to try:
1. Make sure you only have one distribution and one developer certificate, plus one adhoc and one normal mobileprovision related to your game.
2. In your keychain access, verify that you have private keys for the certificates before making the p12.
3. Then, after trying to produce, if it gives the signing error in Stencyl, have Xcode open in the background verifying that your account is the active account.
If you've been working on it for a while (especially clicking the "Fix" button in Xcode), you may have a lot of keys, certificates and mobile provisions floating around.