Hi Valerio,
I guess this is a little bit misleading message but I believe this could also due the fact that the rule is marked as Final - can you check this?
This makes sense, as the Final flag is set once you use the rule in any transaction - and you have said that the system was copied from production.
Check this in table TIMERULEH-FINAL - try to manually clear this flag (for study reason only, to be in accordance with SAP rules, do not do this productively 
) via SE16.
So I guess the official way to change it is to create a new rule and change the date profile.
Regards,
Dawood.