I am also having a variation on this problem. Here is a little more information to support this bug report:
I’m going around in circles trying to connect Version 3.0.0 of the wordpress plugin to our existing AddThis account profile. Everything was working great with the previous version but now am getting errors on the WordPress Addthis home screen (/wp-admin/admin.php?page=addthis_registration).
If I log in using my AddThis username and password, I see the following message: “Your current profile is not associated with this account. You may select a profile from the dropdown…”. I then can see the profile in the list, I select it and click “Confirm”, and then the page stalls on “Creating API Key”. The corresponding message in the browser’s js error console is “/wp-admin/admin-ajax.php 400 (Bad Request)”
If I try to connect the account manually using the correct profile id I get a “Unknown PRofile ID” error and can’t continue.
I’ve tried deleting the plugin and its data, reinstalling and setting up again, and still the same errors.