Apache and SNI - problems with some clients

Never change a running system. Old but true saying, but sometimes there's no other chance. Until a few days ago I was happy with SSL vhosts running with a single SSL certificate. Then I needed to add another SSL certificate for another site with several subdomains like svn.site-A.de, trac.site-A.de and www.site-A.de. With Apache2 running on Squeeze it's possible to make use of Server Name Indication (SNI) mechanism in order to be able to use multiple SSL certs on a single IP based vhost setup.

Well, it works for some client software, but apparently it does not work well with korganizer or Firefox Sync plugin nor with Cyberduck on OS X. Here's an example config: 

SSLEngine on
SSLCipherSuite ALL:!ADH:!EXPORT56:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP:+eNULL
SSLCertificateFile  /etc/apache2/ssl/site-A-cert.pem
SSLCertificateKeyFile  /etc/apache2/ssl/site-A-key.pem
SSLOptions StrictRequire
SSLProtocol -all +TLSv1 +SSLv3
SSLCipherSuite HIGH:MEDIUM:!aNULL:+SHA1:+MD5:+HIGH:+MEDIUM
SSLVerifyClient none
SSLProxyEngine off

This is identical to all SSL vhosts on my system. The funny thing is now that it works for two sites (site A and site B) while it doesn't work for site C. In Firefox Sync plugin I get an error that the connection couldn't be established while on Cyberduck (a webdav client for OS X) I get a requester stating that I get cert for site A on site C. Pointing the browse to the appropriate URL I get the correct cert for site C on site C.

Is there anything I miss with SNI setup in Apache?