A bug was pointed out to me in the current release of WP-Members (2.5.3). The random password that is being sent to new users when they register is invalid.
This bug is the result of some changes that were made to the registration function in this release.
If you are using WP-Members 2.5.3 on a production system, it is recommended that you roll back until a fix is released.
Since this is a pretty major issue I am giving it top priority and will have an update out today to correct it.
I appreciate the very solid userbase that this plugin enjoys and I always strive to keep the plugin up to a professional standard.