In the meantime I’ve successfully performed a rollback to 0.84.6. The error is now gone and the automation is working again, but I can’t stay forever on this version.
I did a clean install into a new VM. Clean config of everything except the onkyo media_player component. Worked just fine in every version > 0.84.6 I tried. Must be something with our configurations.
I still haven’t nailed down the issue in my live setup.