@Mol123321_NL
If the username/password is incorrect, then that means you are trying to login on the wrong version of OSM. With OSM NL (Dutch version) and OSM World (international version) we have two versions of OSM that act independently from each other. You can have an account on both versions, but they only work on the version where the account was created on.
To login to the Dutch version you will have to switch the region. On Android you can do that on the login page of the OSM app.