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I was looking into this issue and found that Safari's "Prevent Cross-Site Tracking" feature—and the similar option in Chrome—can cause it. Open Safari, click Settings in the top menu bar > Privacy, then find and uncheck the Prevent cross-site tracking option. Close and reopen Safari, then test it out. You can also check the step-by-step instructions at the link below.
http://support.apple.com/en-us/guide/safari/sfri40732/mac
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