[_ck_]

You’re possibly getting 404’s only in IE because of session issues in Internet Explorer.

bbPress doesn’t use sessions by default so some other plugin must be putting them in (I have one that does and so does bb-post-views and possibly a few others).

IE is *very* picky about how sessions are done.

What plugins are you using?

Also, are you using multiviews in the .htaccess for bbPress? That could cause some issues. If so, use the real rewrite rules instead.