I have a standard install using 3.9.1 Emphaino theme, and I set up with the permalinks set to the standard /p=?? where ?? is the ID number of the post.
I am trying to change this to show permalinks as /p=%postname% so I installed your plug in, activated it and then changed the settings in the Permalink section to use /%postname%/
However when I do that, all the links break and show the error message above.
