In 11.2014 I made a payment and MultiBit generated new address and sent rest of my bitcoin there. I wasn't aware of that and kept using my wallet thinking that my bitcoin is safe.
I upgraded to windows 10 in last august and with my .key file I was able to restore only access to my first address. Which was empty.
So that means there is no way to access the new address? Thanks!
I have had a similar problem. I was playing around with it right now and it seems you need to back up the .wallet file along with the .key file. It is because when you import the .key file, then it gets imported to a new wallet which already has an existing address. This creates 2 addresses in one wallet and funds often get transferred from one to another during a transaction.