I've tried going back to firmwares v1.08 and v1.07 on one of the DCS-930L's, on the theory that the things about FTP that got fixed in v1.09 might have broken something else - but no such luck.
1and 1, my server provider, has replied to a series of emails - each time from a different person - so we're running around in circles. Nothing at all heard from "help@dLink.com". But, one useful data point is that using a specific numeric IP for the server, rather than the domain name, with NO other changes, made ftp work for 2 DCS-930Ls. I'm not sure that that is a permanent solution.
Sure is very odd that a DCS-931L can ftp to host = named domain, Path = /foldername while the DCS-930L's can not!
I should add, I did also try nested folders. And that it isn't only the "new" DCS-930L like the thread title started out.
Any ideas out there? Drowning here.