I have no problem with legitimate feedback and reviews. That is what makes the plugin better. However, this user has made some claims that do not appear to be of the norm. Unfortunately, they have provided no legitimate feedback which makes the comments and review essentially useless to me and to others.
The only thing that I can surmise from this description is the possibility that they used the plugin with moderated registration (admin approval required) turned on and sent the password out of sequence, thus resulting in an invalid password. Following setup instructions included with the plugin and freely available at the support site would have avoided that issue (if that is what happened).
A broken database is unlikely and that is the real issue I would have liked more information on. WP-Members does not make changes to the database in the sense that it does not create, remove, or edit and tables. It uses the wp_options table to store its settings. It uses wp_users and wp_usermeta to store user information using WP's own functions for that.