I'm no expert but this type of error comes from the engine attempting to load something that has been deleted. The most common occurrences of this I have seen, are troops trying to use an enemy that has been removed from the database. If you have recently decreased the number of entries in any category of the database; items, enemies, states or any others, I would try increasing it.
This can also apply to events, it some process is trying to reference an event that is no longer present.
Look for any process that may be trying to call data that is no longer present. As far as help goes, I know it's pretty vague, but it's a decent starting point. Maybe alos check the RPG Maker Forums for similar issues or post there for help.