KS
I would make sure the key that "doesn't open the backbox" is indeed an incorrect key. Check the number on the lock vs the number on the key. If they are the same, your lock might be hung up. Don't want you to throw out a key that might actually work with a little elbow grease.
Edit: from the pic, it looks like the number on the key is different from the lock that it is inserted into (H95 vs E21), so it is not the correct key. They are easy to pick and replace, or you can have a key made. Don't drill it unless you have to.....too many metal shavings and too much chance of damaging something.
