Hi Vanja,
I really like to help you but currently I have no idea what is wrong. One thing which makes me wonder is that your log misses a call to Application.SetMute - which will be send to Kodi once you try to mute it. How does your configuration for the mute channel and item look like?
The API should be compatible - at least for the mute feature …
One more thing: May I ask you to put the logs into code fences? Thanks.