Sony just posted a notice on the official Japanese website of Bloodborne, advising players against joining multiplayer via the Small Resonant Bell or the Sinister Resonant Bell while in the Forbidden Forest.

The bug may prevent the Lunarium Key to appear in the Byrgenworth College, thus blocking players from continuing the game.

The problem doesn't appear when taking part in multiplayer as the host, but of course you're still luring other players into getting the bug, so you may want not to do that anyway.

The notice also mentions that a hotfix is in the works, and will mend the issue for those that incurred into it as soon as it'll be downloadable.