I'm getting the same intermittent error with 9.1.1 (2328). In
previous versions, I could quit, come back and open the file from the
server OK. That doesn't work with 9.1.1. However, I may have
discovered a workaround. When the error occurs, dismiss the dialog,
close the document, close the FTP browser, open a new FTP browser and
open a file from a different site. Close that file, disconnect, and
then connect to the original site and try to open the file. Anyhow,
that worked once. Since then, no errors again yet.