Welcome to SE! The error is about renaming files which suggests a permissions problem. Check the permissions on the templates_c folder and remove all the existing files in there (they will be recreated automatically).
Is this a new site? If so, how did you do the installation? Permissions should normally be taken care of by the installation process.
The answer is that the meaning of that button changes depending on how the registration is set up and the text of the button doesn't change in line with that, unfortunately. I have a pull request in to fix this in future versions.