I've found the solution at https://github.com/authts/oidc-client-ts/issues/1650.

There is a parameter which can be set in the UserManagerSettings:

const settings: UserManagerSettings = {
    authority: ...,
    client_id: ...,
    redirect_uri: ..., 
    revokeTokenAdditionalContentTypes: ['text/plain; charset=utf-8'],
};