Just posting this because I've spent a couple of days trying to get my ldap login to work again. There a a couple of posts in the old forum that this solution may fix, but since they're read only I'm posting what worked for me in the new forum.
I had 2 servers running SilverStripe with ldap working just fine. One of the servers (my test server) I decided to update to the latest version of Ubuntu (I was behind many versions). I did a clean install which meant I'd have to start from scratch. I got apache2, php5, mysql going without much problem. I reloaded my web root directory that had my Silverstripe installations and they worked just fine. Fine, until I tried to log in to one of them using ldap.
I would enter my ldap ID and password and got nothing but a blank screen. I could long in with email and password but, auth_external wasn't working.
Comparing my PHP environment between my working and broke server I saw that the working site's php environment had an LDAP module.
There's a package called php5-ldap that I needed to install. After a restart of apache, all was working again.
Hopefully this helps someone. It could have save me a couple of days of hunting around. Maybe it should be listed with the requirements for the auth_external module. I read the README, but it didn't list this as a requirement.