Hi Hiroshi,
I have looked into this some more, and it turns out that there is a problem with UNC network paths, as a result of a recent change to the sqlite db that we use. A fix for this will be in next weeks release, but in the mean time you must either use local workspace files, or a mapped drive letter for your network path. You can do this by navigating to your network path in Windows explorer, right-click Map network drive..., and assigning a drive letter. You must then access the workspace in terms of this drive (not a UNC network path of the form "\\...").
Sorry for the inconvenience,
Matt