It's a Windows 7 (64 or 32 and 64, not sure) Permissions issue -- I'm pretty sure. It may be an ongoing Microsoft Windows/Office permissions issue bug too.
BUT, what works for me is (1) give my login account "administrator" privileges, and (2) make sure the "properties" for the folder you store your templates (including "Bookcreator") and normal.dotx, etc. gives full read/write privileges to administrator accounts. That takes away the annoyance of all of those "can't save" etc. messages for me. In all cases though, the epub book was created by Bookcreator.
Microsoft is such a second-rate, do the job half-way company, no wonder Apple is such a success. . . . :-( (I still to Windows instead of Linux or the Mac simply to keep to one OS/set of apps on all computers -- life is simpler -- if a big, big hassle most of the time). Bookcreator *IS* great, and I use it quite a lot and over many of its iterators!