nginx_mail_proxy authenticate to imap_ssl upstream ssl

[tom]

Hello list,
I configured sucessfully the mail_proxy for nginx 1.10.2 von RHEL7, but authentication only succeeds if upstream server which is provided by the auth_http Server is cleartext, e.g. if the auth-server responds
2017/07/20 11:02:47 [debug] 9535#0: *49 mail auth http header: "Auth-Status: OK"
2017/07/20 11:02:47 [debug] 9535#0: *49 mail auth http header: "Auth-Server: 192.168.0.200"
2017/07/20 11:02:47 [debug] 9535#0: *49 mail auth http header: "Auth-Port: 143"
then everything works fine, but having
2017/07/20 11:02:47 [debug] 9535#0: *49 mail auth http header: "Auth-Status: OK"
2017/07/20 11:02:47 [debug] 9535#0: *49 mail auth http header: "Auth-Server: 192.168.0.200"
2017/07/20 11:02:47 [debug] 9535#0: *49 mail auth http header: "Auth-Port: 993"
2017/07/20 11:02:47 [debug] 9535#0: *49 mail auth http header: "Auth-SSL: on"

I get 
2017/07/20 11:03:47 [info] 9535#0: *49 upstream timed out (110: Connection timed out) while connecting to upstream, client: 192.168.0.200, server: 0.0.0.0:10993, login: "user at domain.com", upstream: 192.168.0.200:993
When I directly do a 

openssl s_client -connect 192.168.0.200:993 -crlf
I am able to login with
. login user at domain.com password

Any help is appreciated.
Thomas

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