When I try to open en.wikipedia.org I get following error: "The server's security certificate is revoked!". So far as I check this happens with both Opera and Internet Explorer, but only with this website. Could anyone point me at least where I should look for solution?
The issuer of the certificate unfortunately messed something up so that it might happen that certificates get reported as revoked which are not revoked at all. See GlobalSign screw-up cancels top websites' HTTPS certificates for more details.
As noted by Steffen Ullrich this was caused by an error by GlobalSign. If the affected websites do not take action, you could be blocked for four days.
Use the following command to remove the erroneous entries from the Windows Cache
certutil -urlcache * delete