This is an Opera problem, not an OpenDNS problem.
I have the same problem and when I don't use Opendns servers it works fine. Changing to different browsers does not make a change.
I would think that you have the domain blocked with your settings, especially if the block page appears, so don't block it. If you need further assistance, you should at least post the domain name in question, ideally also the output of a related DNS lookup.
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