The Zotero sync server did not accept your username and password
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The Zotero sync server did not accept your username and password.
Please check that you have entered your zotero.org login information correctly in the Zotero sync preferences.
That message generally means what it says. Clear the username and password fields completely, restart Zotero for good measure, and then try again. Also make sure you're using the latest version of Zotero (currently 5.0.33), though that shouldn't matter.