@french imp - there is nothing I can think to do about it, its a choice between two evils. The problem is non UTF8 characters in your path (your library must include them somewhere in its path, even if you are using english books). The change I made in the past was to to change the way Open With works and use a different API call to launch the external process which could cope with non UTF8.
However in Sigil 0.6 user_none made changes to his build/release process which have upset subsets of both Mac *and* Windows users. The problem for Windows users is that using his release builds of Sigil it has a conflict with the C runtime dlls between what is used in Sigil and what is used by calibre. The only way I managed to hack this as a workaround was to hard-code a special exemption for sigil.exe inside this plugin to use the *old* way of launching Sigil, which solves this issue.
However as you have found, that reintroduces the problem some foreign language users have where the path contains non utf8 characters...