Hi, I had the same problem with some plugins, the only way to install them properly was to revert to an older version (0.7.45) of Calibre. My Calibre data is stored in C:\Users\Loeffel\AppData\Roaming\calibre so only ASCII is used. So the error must be somewhere else. I have a german Win7, but as you know Win7 only uses english pathnames and only translates them for the user. So the path is shown as C:\Benutzer\Loeffel\AppData\Roaming\calibre.
The plugins where easily installed with the new plugin downloader. When I had the problem the actual version was 0.7.50. I had tested many thing then.
Some plugins could be installed with 0.7.48, other with 0.7.48 but not higher. The higher the Calibre version was more plugins couldn't be installed any more.
The plugin updater I could install with 0.7.45 perhaps with 0.7.46 but not with 0.7.46 or higher. After installing the plugin I've updated Calibre und then I was able to install all plugins via the updater without any problems.
I've tested many things unzip and zip the plugin, download it with various browsers and downloaders, with and without virus scan, clean Calibre installation from scratch, the result was always the same the higher the Calibre version less plugins could be installed.