Changing block configuration creates new empty blocks

I created a block with the tags for my user account, that worked fine. I then wanted to change the number of links that were shown, so i went to admin/build/block and clicked on the configure link next to my new block. This brought me to the block specific settings. I changed the number of links to 5, but the block didn't change.

I went back to that block configuration page, but the link count was still set to 10. Then I went to admin/build/block/delicious and noticed that besides the block that I created first, a new entry appeared in the list, one without a name (and impossible to delete).

So instead of modifying the existing block, something new was created instead.
Note that if I edit my block on the admin/build/block/delicious page, the changes are registered correctly.