When trying to setup the LDAP Authentication in the ACP, there is a field named "LDAP email attribute".
The correct value for this field is the name of the email attribute, which is a simple string (in most cases, it will be: "mail").

Entering this value, generates an error message of the browser, saying that the email address supplied is illegal, since it doesn't contain a "@".
This error, does not allow submitting this form.
See attachment for a screen shot of the error message, from Chrome browser.

This issue occurs in version 3.1.0-RC1, and all the previous versions.

Possible solution for this issue can be changing the field input type from "email" to "text", in adm\style\auth_provider_ldap.html (around line 25).