I also ran into this after updating to 2.5.7.
When a user was logged in, they could see the site as expected. When logged out an internal server error appeared.
It also happened if bbPress was the only plugin active on the site. The site has been running bbPress without incident before this for a couple of years.
Reverting to 2.5.6 does resolve the issue at least until an update can be made.
