Opera browser can't load twitter CSS: OCSP Error. (2013)

Opera browser is having a problem loading twitter, since today.

It refuse to load twitter CSS.

twitter's CSS is this: http://abs.twimg.com/a/1382083649/t1/css/t1_core.bundle.css

when loading it, the error message is:

Unable to complete secure transaction

Secure connection: fatal error (1066)

https://abs.twimg.com/a/1382083649/t1/css/t1_core.bundle.css

Unable to verify the website's identity (OCSP error).

The response from the online certificate validation (OCSP) server was too old.

Check that the address is spelled correctly, or try searching for the site.

this problem started to happen on 2013-10-20.

OCSP is Online Certificate Status Protocol, a protocol to check if a certificate is still valid. (certificates are used for secure connection (https), see: Understanding Public-Key Cryptography for Beginner.)

It seems, either twitter's certificate has problems, but Firefox and Google Chrome doesn't have the problem.

OCSP is enabled in Firefox by default. See: http://kb.mozillazine.org/OCSP_error_when_accessing_secure_sites

There does not seem to be a way to disable OCSP in Opera.

Nor does it seem possible to turn off https in twitter today.

The problem disappeared today.