I was chatting to a colleague this afternoon (we work for Symantec UK support). We have had a few cases with FSA Scanning and they have been tricky. One issue we came across was possibly due to the IIS Timeout (which is 20 minutes by default).
You could try increasing this (on the EV server) to say 1 hour and then test again to see what happens.
Nice link here showing you where the value is http://support.yessoftware.com/kb_article.asp?s_keyword=.gif&s_prod=&kb_articlesPageSize=10&s_type=&order_by=Freshness&s_cat=&article_id=74