The subtitle is either supposed to show the current reading list or whatever the #fanficcat is. If it's neither of those two it should show nothing.
This is the template. It shows no error in the editor.
Code:
program:
if field('#readorder') then
template('Reading List: {#readorder} #{#readorder_index:0>2s}')
else
if field('#fanficcat') then
template('{#fanficcat}')
fi
fi
Could it be a bug in the KoboTouchExtended drivers?