WP 3.0.4. multi / BP 1.2.7 / Ultimate Translate 1.0.6 (in /wp-content/plugins, Network Activated as a Supporter option)
When I upgraded to WP 3.0.4 I had some permalink errors. (see my other post on permalinks and Simple Press Forum). Suffice it to say, I went from a date / post name to a numeric link to get things to work again. After all that, I discovered that there were many old posts that would return a 404 error, or weirder yet, would redirect to another Page.
The post would show the correct NEW permalink on the backend, but visiting or viewing the post, as well as directly entering the permalink or post ID link returned an error 404 / blank page telling me that cookies may not be enable or to notify the admin.
I scoured the web, tried all sorts of things (all of which had already been applied), pulled out my hair and then started to cut/paste the old posts into new ones, and then delete the old ones.
Then it dawned on me, it wasn't every post that had this problem. So I put each post (good one vs bad one) side by side to see what different. It turns out that any post that used the Translation plug in on was a dead one. If I toggled the post to "Do not translate this post", voilà the post would become available again!