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.