linux4me2 - I copied and replaced the whole code you posted.
I went into permalinks and changed it to show the post ID, and when I click on 'Wordpress' for the none-working posts it does say ID = 3.
I've also checked Posts --> Categories, and there is definitely only one Category called 'Wordpress'
All posts only have 1 defined Category, some have 1 or 2 tags.
I'll try making a new post with the same content and see if that works.