Permanent notification after failed Imgur upload

[Lucki]

My screenshot upload failed, I don't know why because the next one seconds after that worked just fine.
However, I now have a permanent persistent sticky notification always on top - even above fullscreen applications. I can't get rid of it besides rebooting (hopefully that'll do it).

• Hovering over the notification reveals a tiny X which normally closes notifications but doesn't on this one.
• Clicking on the notification itself does nothing.
• The notification is not among the other notifications in the notification area when clicking on the date and time button.