It's possible the change is not being saved to disk ... and is only there in memory.
If you restart your blog, you can see if the changes are still there. You can restart your blog by making any change to the web.config file (add a space, etc). The blog will automatically restart by itself too once in a while ... if it's inactive
for a while, or the application pool recycles, etc.
You can also check that XML file in the location indicated in the error message to see if the changes you made to the post are actually there.
Generally that error message (Access to the path ... is denied) means Write permissions are not enabled for the ASP.NET account. BE needs Write permissions in order to save data to the App_Data folder.
Is this an existing blog and the error just start happening? Or is it a new blog and you've been seeing this error since starting it? If it's a new blog, and you're seeing this error message, then probably Write permissions are not setup for
the App_Data folder.