I have an installation of DNN 4.0.2 running for a client to play with while we consider moving all his sites to a DNN platform.  This installation has worked just fine up until yesterday when suddenly every user we try to log in with (including the superuser) generates the login error of:

 "Login Failed, remember that Passwords are case sensitive" 

The installation has connection to the database because it displays all pages and even allowed us to create a new user.  We went into the DB to authenticate this new user and set him as a superuser but upon trying to log in - got the same 'login failed' error.  We have restarted the site via IIS and have even re-saved the web.config file so as to make the site dump it's cached info and use the most recent.  None of this seems to have helped and to my knowledge nothing has changed in the last day to cause this (although obviously something has changed!).  Anyone experienced this before and if so, what was your fix?

UPDATE: We were finally able to get access to the account through another superuser account.  Using this account we can create additional superuser accounts.  The superuser accounts we create fresh can log in, but the previous superusers as well as admin users or below still cannot.  Even if we create a new admin user they cannot log in and receive the error message mentioned above.

Could the machine keys in the web.config have been modified somehow (e.g., was DNN upgraded without copying across the keys to the new file)? That would account for old passwords not working while newly created ones would work fine.