Copy from Opera issues site (by grnch):
1) Install "Reading List" 1.5.1
2) Click on the button to see your reading list (in order to provoke the login window and enter your user/pass)
3) Go to extension preferences and disable "Show unread count"
4) Restart Opera
5) Click on the button agan
Result: empty window appears and the extension becomes unusable.
If you now select "Show unread count" and restart Opera again, the extension starts working again.
Using Opera 11.11