Cannot Activate OC because only a blank screen comes up when using the Activation Link. this is within 15 day time period.

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Unable to login and set up password in OpenClass. I have several students who have not activated their accounts. I resent their account activations, however, when they click on the link in the email, the screen comes up completely blank. I had them clear the cache in their web browser, and insure that Popups are being allowed for the OpenClass address. I have even had them switch to another browser. Still NOTHING. I don't know what else to do. They are about to drop out of the courses. Can you help me help them?