Curl fail to retrieve HTTPS content

Curl fail to retrieve HTTPS content

i've an issue on my production server running on Debian 6 (apt-get
update;apt-get upgrade done with success).
When I call an HTTPS (StartSSL) server I got an error message on Debian 6 :
#> curl --version
curl 7.21.0 (x86_64-pc-linux-gnu) libcurl/7.21.0 OpenSSL/0.9.8o
zlib/1.2.3.4 libidn/1.15 libssh2/1.2.6
Protocols: dict file ftp ftps http https imap imaps ldap ldaps pop3 pop3s
rtsp scp sftp smtp smtps telnet tftp
Features: GSS-Negotiate IDN IPv6 Largefile NTLM SSL libz
#> curl https://mydomain.com/api/
curl: (60) SSL certificate problem, verify that the CA cert is OK. Details:
error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify
failed
More details here: http://curl.haxx.se/docs/sslcerts.html
But when I call the same address on Debian 7 (server is up-to-date too),
everything working well :
#> curl --version
curl 7.26.0 (x86_64-pc-linux-gnu) libcurl/7.26.0 OpenSSL/1.0.1e zlib/1.2.7
libidn/1.25 libssh2/1.4.2 librtmp/2.3
Protocols: dict file ftp ftps gopher http https imap imaps ldap pop3 pop3s
rtmp rtsp scp sftp smtp smtps telnet tftp
Features: Debug GSS-Negotiate IDN IPv6 Largefile NTLM NTLM_WB SSL libz
TLS-SRP
#> curl https://mydomain.com/api/
{'code':'ko', 'message':'ERR_AUTH_BASIC_NEEDED'}
Do you have any idea why HTTPS fail on Curl Debian 6 and works with Debian
7 curl ?