unsuccessful ad-authentication (squid_ldap_auth) with underscores in username?

[Tom Tux <tomtux80@xxxxxxxxx>]

Hi

I have trouble with authentication some ad-users, which containing a
"_" (underscore) in their username. The helper will always deny such
requests. Users containing no underscore in their username are
authenticated successfully.

Any hints?
Thanks  a lot.
Tom