To play online with others on this specific version, everyone in the lobby must have the same "checksum" (the version code). Right-click in your Steam library. Select Properties > Betas .
Because v1.9.1 is a legacy version (modern versions are 1.14+), the "proper" way to get this specific build is to use the official version-rollback feature provided by Paradox Interactive. Step 1: Purchase and Installation To play online with others on this specific
Are you trying to run a specific that only works on version 1.9.1? To play online with others on this specific