I get that message occasionally as well. But not very often, maybe once a month. No idea what is causing it. Usually it says something about the certificate not matching the location: scratch.mit.edu instead of scratch.media.(something).(something).
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I found the same problem, and I half-know what the problem is: a website called https://scratch.mit.edu/ (might not want to try clicking that) exists, but I've only found it while searching the website (is that where you keep finding it? tell me if not). It starts with https instead of http, and here's the part I don't understand: I always find it, but I don't know if it's made by someone else or just a website that appeared out of nowhere. I posted a topic on this a long time ago, but I never got any responses.
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The SSL certificate was purchased only for the scratch.media.mit.edu domain not scratch.mit.edu (long story).
The only reason why we bought the certificate is for the credit card transaction that happens when purchasing a Scratch Board at https://scratch.media.mit.edu/pages/scratchboard-purchase
Do not worry about that error. If you don't want to be bothered by that kind of errors just go to http instead of https.
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I have that problem too: http://scratch.mit.edu/forums/viewtopic.php?id=5126
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