The issue may be with the Windows client itself. Try to uninstall the client, accept the dialog asking if you want to delete the profiles, and then reinstall. For me this worked.
I also tweaked my DNS server to point to the root domain by default (instead of www), but I don’t think this was the culprit.
See also: [SeaDrive Bug]; Work-around after switching to https - #2 by germeier
The fixes in this thread did not help. I kept the default configuration (adapted to SSL), and it now works flawlessly.