I have been playing all morning with this … first of all the thing is working as it should – so no error!

What came in to my mind is that I had the CE version of metaPOST setup with WLW and after installing the PE version I did not remove the reference from WLW just tried to do a refresh of the blog theme (could be one reason).

Another thing after deleting the Blog reference from WLW and trying to setup a new with the same blog the error repeated it self – I’ve done this several times, then it hit me! I’m on MS Windows! If you uninstall something you have to restart!
Well I didn’t restart, what I did was I went to the AppData/Roaming/Windows Live Writer folder and there it was:
     the blogtemplates folder loaded with various coded folders which represent the settings of all ever subscribed blogs!!!

So the problem is in WLW itself: If you delete a blog reference from WLW it’s settings don’t get deleted!!!

So I’ve gone a step further deleted the reference to a blog – restarted the computer and the blogtemplates for the deleted blog were still there !!! So the resolution to various issues regarding to changed settings or updating a blog with metaPost:
 

1. Always delete the reference to a blog, forum or similar (used with metaPOST) from WLW
2. Manually delete the folder: AppData/Roaming/Windows Live Writer/blogtemplates/(some numbers) (on Vista Ultimate, for other Windows systems try search for "blogtemplates" folder) witch one to delete is hard to say try to move them to another location or something like that, if you delete those folders you will have to refresh the themes for all your configured blogs 
3. Restart WLW and setup your metaPOST account again
4. Problems should be solved

If you have problems like this with other blogs an WLW try this solution mybe it helps ...