I think there is a bug as I have a similar problem, which I fixed.
My problem was that my password _stopped_ working. It's the same password for the same file as I use on PC and iOS synched with DropBox. I even checked the hash was the same for that file.
So I installed Keepass2Android .. and the same password worked. Then I turned the phone off and on (Nexus 5 btw) and KeePassDroid worked again. So try each of those options and hopefully you'll get in.
There's plainly an issue under some circumstances with the app.