When I click the “Edit” button in an opened event, I get the message “Exception has been thrown by the target of an invocation.” I can click “OK” and get the edit window to show up, but when I check the “Post event entry” box and then click “Next”, I get another error message: “Object reference not set to an instance of an object.”
Help, please! :)

P.S. I tried reinstalling WER, but the same thing happened. I also got the same error messages on another computer.

The Judge Program does not maintain WER. Your best bet would be having your store contacting their WPN rep, or contacting Wizards CS at https://wizards.custhelp.com/

Note that this seems to be a common problem, so I'm sure the developers at WotC are already aware of the problem. In the mean time, if you need to edit an event, your WPN rep can help you with that.