I'd call your IT staff on it, but:
http://forums.notesnet.com/forums/forums.nsf/alert?openform&f=b2da9437a7e1fbef85256840001b15d4&parentunid=31ec7317616d7d9d85256c7e001edc55 notes that a missing signature triggers it.
Read http://www-1.ibm.com/support/docview.wss?rs=0&q=file+does+not+exist&uid=swg21093980&loc=en_US&cs=utf-8&cc=us&lang=en as well.
Whenever I try to create a new memo or reply, I get a "file does not exist" error, even though i'm not attaching anything. This started after I attached a file a few weeks ago. When I reply to all it gives me the same message and only puts the original sender in the to: field. Any help would be appreciated.