[Exim] String expansion in fixed-login auth

I had this client-side authenticator
fixed_login:
driver = plaintext
public_name = LOGIN
client_send = : log1 : pwd1

This works well. Now I want the client_send to define as string expansion
depended on $host. But the following string definition does not work:

client_send = ${if eq {$host}{my.relay.de}{ : log1 : pw1 }{ : log2 : pw2}}

Exim's error message:
expansion of "${if eq {$host}{my.relay.de}{" failed in fixed_login
authenticator:
missing } at end of string

I tested some other versions, e.g. escaping the colons, quotation of " :
logx : pwx ".
All don't work.

Any help?

Best regards.
Rudolf
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Rudolf Kompf                     | EMail: Rudolf.Kompf@???