its probably some path related issue with being on a non english windows machine (windows' multi-language support is a prehitoric disaster).
do the following:
make sure c:\calibredb exists and can be written to by the user. then at a prompt run:
calibre-debug -c "from calibre.utils.config import prefs; from calibre.library.database2 import LibraryDatabase2; path='c:\\calibredb'; prefs.set('library_path', path); db = LibraryDatabase2(path); db.refresh()"