Yup, multicraft, I think I figured it out though. Update your 1.4.4 jar file to the newest Dev build. It did the job for me.
(guessing) I think a sheep tried to breed, and the server crashed. That is supposed to be fixed in one of the dev builds.
You're right, I just noticed this line [Server] SEVERE at net.minecraft.server.EntitySheep.createChild(EntitySheep.java:227)