To be more precise, you should disable the dummy imports already in the Base run (the deterministic Current Policies reference scenario), in order to make sure your Base prices are not polluted by the impact of the dummy import marginals. And then of course keep them disabled in the stochastic run(s).
And further, it is also advisable to disable elastic demands in the first periods (e.g. up to 2020), by setting either COM_VOC or COM_ELAST to zero in those first periods. That will give additional assurance that any bad prices that may often appear in the first periods will not cause numerical problems. After all, you cannot expect historical demands to be changed by your policies, can you?
And further, it is also advisable to disable elastic demands in the first periods (e.g. up to 2020), by setting either COM_VOC or COM_ELAST to zero in those first periods. That will give additional assurance that any bad prices that may often appear in the first periods will not cause numerical problems. After all, you cannot expect historical demands to be changed by your policies, can you?