EDIT: UPDATED 2/17/17
Hi y'all,
Our development team is currently investigating an issue where some users are having trouble logging into the Evernote Web Clipper in Safari. When this issue occurs, a tab or window opens and closes and an error appears stating "You must sign in using the Clip to Evernote toolbar button before you can make clips."
If you're experiencing this issue, please ensure your Evernote Web Clipper is updated to version 6.11* and your Safari Privacy settings are set to either "Allow from websites I visit" or "Always allow" (Safari > Preferences > Priva