[exim] Problems with AUTH LOGIN

I am using exim 4.60 with multiple authenticators, one of which is the
LOGIN. It seems I have some wrong configuration since a client dose
never get the expected answer from the exim server when issuing the
command "AUTH LOGIN". Following is some trace from the server:

14659 SMTP>> 250-JUPITER.LTEC Hello dhcp-021.ltec [192.168.1.21]
14659 250-SIZE 52428800
14659 250-PIPELINING
14659 250-AUTH CRAM-MD5 PLAIN LOGIN
14659 250 HELP
14659 SMTP<< AUTH LOGIN
14659 SMTP>> 334 VXNlciBOYW1l
14659 SMTP>> 334 UGFzc3dvcmQ=
14659 search_open: lsearch "/usr/local/exim/auth.passwd"
14659 search_find: file="/usr/local/exim/auth.passwd"
14659 key="" partial=-1 affix=NULL starflags=0
14659 LRU list:
14659 :/usr/local/exim/auth.passwd
14659 End
14659 internal_search_find: file="/usr/local/exim/auth.passwd"
14659 type=lsearch key=""
14659 lookup_login authenticator:
14659 $1 =
14659 $2 =
14659 expanded string: no
14659 SMTP>> 535 Incorrect authentication data
14659 LOG: MAIN REJECT
14659 lookup_login authenticator failed for dhcp-021.ltec (mars) [192.168.1.21]: 535 Incorrect authentication data

And this is the the authenticator from my configuration file that is
supposed to be used to handle "AUTH LOGIN":

lookup_login:
driver = plaintext
public_name = LOGIN
server_prompts = User Name : Password
server_condition = ${lookup{$1}lsearch{/usr/local/exim/auth.passwd}{${if eq{$value}{$2}{yes}{no}}}{no}}
server_set_id = $1

From the trace I can see that the client issues "AUTH LOGIN".
According to the configuration exim should answer with "User Name" but
it returns "334 VXNlciBOYW1l". This is not expected by the client and
treated as error.

Does anybody know where the error in my configuration could be?

many thanks, Felix

PS: Cram MD5 works perfect