failed to get files from server

 Hi, My users frequently complain of an error trying to open attachments to various objects, "failed to get files from server" we are an on demand customer, the file is fine (not corrupt), it's not checked out or already open, it's typically a PDF or word file.  the only remedy seesm to be to wait and come back to it after a few hours and try again.  any help is appreciated.  thanks.