As reported at LaughingMeme, there is some sort of bug or change in PHP 4.3.2 that causes FoF to fail with an out of memory error. Hosts around the world are starting to upgrade to 4.3.2, so you may be hit with this soon. I’ll debug as soon as I can.
The problem seems to occur only on some feeds, but on those, it happens reliably. One example is Aaron Swartz’s weblog.