Opera has the same complaint about the first four public Zimbra sites I tried... using 3 different CAs.
I get Connection : TLS v1.0 128 bit AES (DHE_RSA/SHA) and looking in Advanced/Security/Security Protocols I see SSL/TLS1 128 bit AES (DH_RSA/SHA) checked.
Hmm, if I tell Opera not to allow 128-bit AES, then it successfully negotiates 128-bit RC4, and does not complain. So I guess if you tell it to use AES at all, it demands 256-bit. Opera even complains if you UNcheck 256-bit AES, allowing only 128-bit AES.
I see a few end-user complaints about this behavior, but no detailed diagnosis. Has anyone filed a bug against Opera?