Vrenaut74, can you look to see if you are getting a credit for taxes paid to CA on the MA return? Since the full year state (MA) taxes income from all sources, there will typically be an other state tax credit on that return for taxes paid to CA. If you don't see it, you can use screen 52, Other State Tax Credit for Massachusetts and enter in the CA income and taxes paid so that MA will provide a credit.

Hope that helps! Let us know how it goes.