Running mongo with oldstyle config quits immediately without a message

My mongo.conf unexpectedly reverted to the old style:

dbpath = /home/vagrant/backend/mongodata
logpath = /home/vagrant/backend/log/mongo.log
logappend = true
auth = true

And because of that, running the following quits immediately without any error message:

./mongo/bin/mongod --config mongo.conf

I'm running mongo v3.0.0