Two hours later:
I’ ve tried every possible process to include the sensor. (Sensor within 5 cm from the ST hub: Resetting the device; doing excludes in the old and new Smartthings apps; resetting the sensor again once inclusion mode has started; removed battery of the device etc. etc.)
After about 45 minutes, I managed to include it with the new ST App. The sensor was listed as a “Z-wave device”. Changed the DTH but could not get the sensor to configure. Eventually noticed that the sensor did not include in secure mode:
I excluded the sensor again and reset it as well to make sure. Started the process again and only after another half-hour, managed to get it to include again (using the net ST App)
Unfortunately, it once again failed to include in secure mode.
I tried to configure it using various device handlers, but it fails to report the temperature.
I left it for an hour and so far, no new reports from the sensor in the IDE log.
I had since noticed that one of the other Fibaro Smoke sensors had also stopped reporting data (the last report was with the battery at 100% - can’t see when the hub received the last report.) I tried to wake it up by pressing the button, but that did not help. Also tried pressing the button for 3sec. Nothing. (I suspect it last reported soon after I had replaced the battery about three weeks ago)
I have 4 of these sensors at my holiday home as well, so I checked if they are still reporting. Two of them show “Offline”, and a third last checked in 12 hours ago. (I replaced the batteries of all four the last time I was at the house two months ago.)
Think my time with these sensors has, for now, come to an end.