[sudo-users] ldap authentication and local sudoers

[Todd C. Miller]

It looks like you are hitting this bug:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=368297

There are some workarounds described in the related Ubuntu bug:
https://bugs.launchpad.net/debian/+source/sudo/+bug/423252

The simplest one is probably to just install nscd.

Also see:
http://jdbates.blogspot.ca/2013/04/its-crazy-how-much-time-and-effort-one.html

 - todd