set_required_validators_num should check whether the _required_validators argument isnt higher than the current validator_count. Otherwise contracts can end up in a state where, even if all the validators are active and provide signatures, they also wont be enough to validate cross-chain transaction. Currently such check is not presented and manager can intentionally, or not pass higher number than the number of current validators and DoS the cross-chain functionality until another manager doesnt decrease the value:
BaseSettlement.sol
Add a check to prevent even the managers from passing _required_validators higher than the number of the validators:
