Hello Marcos,
I was just able to get my External Database Enrollment working and I did notice that the Role ID is mentioned as optional in the documentation for 2.1, but it is required! I found this by sheer trial and error. Anyway, to debug, open your php.ini file with a plain text editor. This file is inside of your php installation directory. I am using windows so it is in my c:\php
Find a line that says error_log. Make sure it looks like this:
error_log = C:\PHP\php_errors.html
In my case, I decided to output the errors to an html file because the moodle errors come with html formatting. But, you could call it anything you like. Ex: myerror.log
Sign in and then check the file on your server. It should contain some errors if there is something wrong with the connection. However, if you don't get anything and it still doesn't work, you can add the code "debugging('some text here')" without quotes to any file in moodle to see what is going on.