Can't issue any Z-wave service call

I am unable to Heal network or Add nodes or Cancel command. None of the buttons on the Z-wave settings page are responding. Also manually activating a service call like zwave.add_node does nothing. No popup, no confirmation.

The existing Z-wave network is up though. All previously added nodes are present and responding as usual. I am just unable to add a new node.

What to do?

I am running Hassio on NUC with Ubuntu Server 18.04. Home Assistant version 0.91.4. Supervisor version 162.