I have never used absolute paths for Composer repositories.
You could try with a relative path.
Also Windows uses backslash \, maybe Composer is sensitive to that? The opposite could also be true, some tools use only forward slashes for paths even on Windows.
Also maybe there's a protocol required for when using absolute path, for example file://. Did you find a documentation that explains how to use absolute paths in the Composer documentation? I think all examples I've seen until now use relative paths.