My client was seeing an odd error in one of their environments, bot not any of the others. When you would go to configure an out-of-the-box workflow on a site, you’d see the "The form cannot be displayed because session state is not available." error after clicking Next on the first configuration screen (/_layouts/AddWrkfl.aspx).
Some research pointed to several different potential causes, each of which lay in the Shared Service Provider (SSP) settings.
The first one told us to check the Form Session State in the SSP configuration below (/_admin/ipfsConfig.aspx). Clearly a good idea, but no joy. (I didn’t find this one, so I don’t have the source.)
Open the web.config file for the site, search for the word "SessionStateModule" and uncomment that line.
<!– <add name="Session" type="System.Web.SessionState.SessionStateModule"/> –>
<add name="Session" type="System.Web.SessionState.SessionStateModule"/>>
It’s not clear to me how this line would have been commented out in the first place (I have no access to the servers or to Central Administration), but this was the fix that worked.