I am having the same issue. In the Windows Evernote client, if I click on "Tasks Early Access", the tasks window opens, showing the tabs "By note", "By due date", and "Flagged". All I see is a spinning green circle. Nothing is loading, since I can leave it spinning for hours. I only have a few tasks, so it should not take long to load. I have defined the default task note. It seems to me that when I changed the default note from what Evernote set as the default (I believe it was a note titled "Things to do" or something like that), everything stopped working.
