Hello,
When compose calls the LDAP search function (to build the auto-complete list) it passes an empty '$value' parameter. In this case, when all the bits of the filter get concatenated you end up with something like this:
(|(mail=**)(cn=**))
which isn't valid syntax (at least it's not on my LDAP server).
Here's a tiny patch to rcube_ldap.inc that eliminates the double wildcards. There may be a better way to do it, but this was simple.
Cheers, Roy