I am using Register Plus Redux. I don't know what happened exactly but after I upgraded to 3.1.4, the password provided on registration isn't recognized. I have to go in using admin account and set the password again. This does not happen with the default registration form though. I don't where to contact the author, if this isn't appropriate feel free to delete it.
This is a bug in the Register Plus Redux and has nothing to do with other plugins.
Normally when you register with normal wordpress, it sends you a temporary password that you can modify after.. what happens with RPR is it sends you by email the temporary password generated by wordpress but puts in the database the password you have registred with! causing this bug.
I advise you to disable this plugin now while waiting for an update from author.
some users advised to edit the file register-plus-redux.php but this is risky since when plugin is updated it will overwrite your edits.