Previously, when a not logged-in user cliked "Add Reply" in a thread, they were redirected to the reply page for the thread after logging in. This no longer seems to be the case -- the user is redirected to the front page.

In particular, I was viewing a thread in the foobar2000 forums, which I navigated to from the main site.

Yeah. This is an issue we couldn't yet find a fix.. Maybe something related to server configuration or might be ipb bug in certain environment.
Anyway, since this is not critical and we don't have so much time hunting this, it may take some time until this is fixed.

I was able to isolate the problem to the invalid behaviour of IPB when certain PHP configuration setting is in use. Anyway, it seems it's a bug in IPB and I've submitted a bug report at Invision forum.