Has been a great extension, but the latest update has broken it. I have version 4.0.76. Now, when I click on the button, it does not recognise my login at all and shows none of my boards. It gives me the option to create one but the "create" button is greyed out. When I go to the pinterest homepage I am logged in, so I can't find a way to fix the issue. Renders the button totally unusable. Three stars just because up until now it has been great.
Thanks for confirming your version with your report! Are you by any chance using account switching on Pinterest?
Other questions: which button are you clicking, the hovering Save button or the browser button in the toolbar? If it's the Save button, are you seeing a pop-up window with the Pin create form in it, or is the Pin create form showing in an overlay on top of the page?