I am reporting this as a new issue as I have no ctools and I can reproduce the problem consistently with both Firefox and Opera. I defined a new content type: 1 regular vocabulary, 1 HS vocabulary, 3 unrelated CCK fields, everything marked as "required". When adding a node the HS selection seems to work fine, however if I first attempt to preview or save the form without filling any field (just for the sake of testing) then subsequent HS selection attempts will result in Received an invalid response from the server. Please change status as required. Thank you.
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