MinGW doesn't know that the root name in Windows can start with two slashes:
assert(fs::path(R"(\\server/folder)").root_name().string() == R"(\\server)");
assert(fs::path(R"(\\server/folder)").root_directory().string() == "/");
assert(fs::path(R"(\\server/folder)").root_path().string() == R"(\\server/)");
assert(fs::path(R"(\\server/folder)").root_name().string() == "");
assert(fs::path(R"(\\server/folder)").root_directory().string() == R"(\)");
assert(fs::path(R"(\\server/folder)").root_path().string() == R"(\)");