I’m sorry but it’s quite simple. I’ve 3 different wordpress installations, if Wp Insert is active my Feeds don’t show any excerpt and/or thumbnail, i can switch on and off this problem activating and deactivating the plugin ??
I can add that some third party app can still import excerpts and/or thumbnails but they’re no more visibile if, for example, i put the Rss address into Internet Explorer to check it.
(I use Wp Insert since months and never had a single problem with it)