Security Issue with Opera

[Etienne62]

I use Opera as my usual browser but just recently whenever I type in a URL I get a box appear entitled "Security Issue - The server's certificate chain is incomplete, and the signer(s) are not registered. Accept?" I can't seem to get rid of it and it has not happened before.
Any ideas as to what this is and how do I get rid of it?

[galeck1947]

Change to a more secure browser like IE8 Opera is a very insecure one and is targeted quite a lot