The new keyword based block doesn't work correctly

Steps to reproduce

Setup

1. Log in as admin in ezflow
2. Click "Conference" in top menu
3. Create new article "Article1"
Keywords : testsystem, systemtest
4. Create new article "Article2"
Keywords : testsystem, failure
5. Create new article "Article3"
Keywords : systemtest
6. Create new article "Article4"
Keywords : failure

Test the keyword block

1. Go to frontpage
2. Edit fronpage
3. In "Block type", select "Keyword" ( FIXME : Find correct name )
4. Click "Add block" button
5. Edit the new block:
Name : keyword test
Source : "Conference" subtree
Class : article
Keyword : failure
6. Publish the frontpage and validate that the keyword block shows no articles
7. Edit the keyword block again : change source to be "Conference/News" node
8. Verify that following field values are preserved:
Class : article
Keyword : failure
9. Publish the frontpage and validate that article2 and article4 are listed

Step 8 and 9 will both fail
Note that it is important to specify "Conference" as source first (do not specify "Conference/News" right away. Then it will work....