In case this helps anyone... got a response from the Seblod team that the form was not loading because the form action was:
action="/component/cck/"
Solved it by changing it to:
action="/component/cck"
(i.e., removing the trailing slash). Now, I'm not sure why it would inject the incorrect action, but alas, it did (we installed Seblod 2.0 when it was brand new so there's a boatload of Seblod legacy code on our site so perhaps that's why).