It sounds like the connectionString to the Membership database is working, but you may be providing an invalid username or password. You can debug this further by implementing the OnAuthenticate Event:
http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.login.authenticate(v=vs.100).aspx
You might also consider just creating a new User and test logging in with the new username and password vai the Membership.CreateUser method:
http://msdn.microsoft.com/en-us/library/t8yy6w3h(v=vs.100).aspx