A '+' sign in the filename is converted by Shellwords.escape to '+'. That's not running on Windows. No idea how to correct this. Shellwords seems for unix and not for windows.
If a filename contains Umlaute 'ä', 'ö', ... a cmd.encode!('ISO-8859-1') is required on Windows.
The problem with the '+' sign should be solved in version 1.7.0.
Problem fixed in mini_exiftool 2.2.1.