Opening a path like \\server (without a trailing share name and path) produces this error, while opening e.g. \\server\share (for a nonexistent server/share) produces ERROR_BAD_NETPATH (which already is mapped).
This happens in some testcases (in fs.op.proximate); as proximate() calls weakly_canonical() on the inputs, weakly_canonical() checks whether the path exists or not. When the error code wasn't recognized (it mapped to errc::invalid_argument), the stat operation wasn't conclusive and weakly_canonical() errored out. With the proper error code mapping, this isn't considered an error, just a nonexistent path, and weakly_canonical() can proceed.
This roughly matches what MS STL does - it doesn't have ERROR_BAD_PATHNAME in its error code mapping table, but it checks for this error code specifically in the return of their correspondence of the stat function.