I assume you upgraded to a new version of MultiChain from one of the alphas or early beta. If so, you probably started using a version which introduced a check on the blockchain parameters, to ensure that in the long run there will not be a 64-bit integer overflow in terms of the total block rewards over the chain's entire lifetime. Assuming you don't mind starting a new chain, consider making the native-currency-multiple smaller, and reducing the initial-block-reward and first-block-reward accordingly.