Hello Federico,
I'm not sure of your PI release. The below steps are relevant for 7.0. If you have a higher release try following these steps in the NWA, just go to the Key Storage service.
1. Go to VA.
2. Select the service named: "Key Storage"
3. On the right panel select the tab named "Security" (at the bottom)
4. Select the action named: "Read Entry"
5. Select view named: <your view>
6. Select the entry you are using for signing
7. From all domains select: sap.com/com.sap.xi.services/aii_mt_rt.jar
and click the button "Grant"
Regards
Mark