THX for that hint! I have solved the issue!
I seems I really forgot to start the Boblight Server in Hyperion.config.json
Code
/// The configuration of the boblight server which enables the boblight remote interface
/// * port : Port at which the boblight server is started
/// * priority: Priority of the boblight server (Default=900) HINT: lower value result in HIGHER priority!
"boblightServer" :
{
"port" : 19333,
"priority" : 900
},