How to tell if underfloor heating is running - ERR still without thermostat cover

The 5th image shows the overcurrent valve. During the heating period, you should close it at the latest and open all ERR so that the heat reaches the screed.

Actuators are removed. I can switch off the distributor box on the ground floor separately in the power box. These were warm before and are now cold. Unfortunately, this is not possible on the upper floor. I have also removed the actuators there. Do I also need to disconnect them?

No need to disconnect, you can just set the corresponding room controller so that no current flows.