Which is the monthly reward for masternode?
Nothing, it depends on your Activity. You earn the fees from Users which use your "Masternode" as their Remote Daemon to use the Desktop Wallet as a Lightwallet. You earn 0.25% of every Transaction Users send or receive through your Wallet.
My wallet crashes within a few seconds after startup. I've tried modifying the .cfg file but I can't get it to run.
Any sugestions?
Which Version, which OS do you use? We can't help you when you dont give any further Informations.
Delete the .cfg and start new