@Loeffel - that your AppData path does not have accented characters in it matches what I expected as stated above. Otherwise you would not I believe have ever got these particular plugins to work, regardless of Calibre version or fiddling. This is a Python level problem, not a Calibre or a plugin one.
However I believe (and correct me if wrong) that at some point when you got that error you had the plugin zip file that you had downloaded in a subdirectory somewhere which had non-ascii in the path to it?
As to the variety of behaviours you saw, not all plugins use the ZipImporter mechanism. For instance the Goodreads metadata download, Goodreads covers download and a couple of the other plugins do not use ZipImporter.
Last I heard from Kovid was that he is working on a new mechanism for plugins. Once he has that available I guess it will become clearer whether all us plugin developers need to make a change to support that new mechanism.