I've been struggling with the problem the whole day:
I Was using the following configuration, because I wanted to first bind, using the authenticating user credentials, and then perform an ldap search.

This is the configuration i'm using

AUTH_LDAP_BIND_AS_AUTHENTICATING_USER = True
AUTH_LDAP_USER_SEARCH = LDAPSearch("OU=ex1,OU=example2,OU=ex3,DC=example,DC=ex,DC=com",
                                   ldap.SCOPE_SUBTREE,
                                   "(userprincipalname=%(user)s)")

Which caused me the error.

The fix was to actually bind with the authenticating user when AUTH_LDAP_BIND_AS_AUTHENTICATING_USER is set to True and when not using AUTH_LDAP_USER_DN_TEMPLATE but instead using AUTH_LDAP_USER_SEARCH.

Note that I had to add at line #479 because self.dn is not required to be set nor should be read otherwise a binding with AUTH_LDAP_BIND_DN and AUTH_LDAP_BIND_PASSWORD is done.