I just stumbled upon the same bug. It's a bug in php's reflection API (http://bugs.php.net/bug.php?id=38132).
The keys for the managed models array get corrupt when the Object::get_static function uses the Reflection API to get the static vars of an admin class. Thus silverstripe cannot find it using the stat function and thus thinks no managed models have been specified for that admin.
We used PHP version 5.16 as well on that server. We switched to PHP version 5.2 and everything works out fine. So just upgrade and it should work fine.