You have my sympathy had this and other errors with several feeds for several months was hoping the last update would fix them but to no luck RSS Aggregator plugin has no problem with them so must be a bug in feedwordpress.
This is one of the feeds giving a error http://www.rspb.org.uk/community/placestovisit/leightonmoss/b/leightonmoss-blog/rss.aspx
Do not know if this is a plugin issue as it is to do with the database
Is the plugin responsible for writing to the excerpt field or is it wordpress?
Feedwordpress could be irrelevant but will post there as well
