WordPress Membership Plugin
The debug log does not take in account any registration attempt. Something even weirder is the login. The testing account I am using to login is not in the list of the members, nevertheless I can still log-in. But in the login related debug log file, that login is not present. I suspect some database corrupted data.
I am not using any cache plugin. The form of the registration is handled by the SMWP Form Builder.