Like most of the other posts on this forum, I upgraded to 2.0 (turning off cookies is something I've wanted since I first installed Collapsing Archives) but 2.0 appears to be completely broken. I tried to fix it myself, and I tried to find any information on what I might be doing wrong on the author's website, but no luck.
I think the only way to fix the issue is to revert to 1.3.2, the last working version of the plugin.
Since neither wordpress, nor the plugin author, seems to offer older version of the plugin for download, I've hosted a copy of it on my server for others to download.
Normally it's a pretty bad idea to download a zip file from a stranger, but I promise it's safe. You can check out my site if you want to confirm that I'm a real wordpress user.