Yeah, all the previous versions of KindleUnpack should've been barfing on you as well (if you're using it on the same machine with the same user account), because the exact same line of code occurs in each 0.72.1/2 0.73.0 and 0.75.0.
This seems to be a
Python 2.7 bug. os.path.expanduser('~') appears to be broken on Windows when usernames contain non-ascii characters. There also appears to be no interest in fixing the python bug.
Until I can come up with a workaround for the python bug, my only suggestion would be to run calibre under a user account that only contains ascii characters.