Finally found some time to test today: version 2.1.0 did not seem to fix the issue but, running from cli it worked. After a reinstall the “normal” app worked too
Trying to reproduce the issue I found the following: after saving the settings it takes a couple of seconds before the app connects to the alarm … when you press read before it has connected it fails (and subsequent tries now fail too). Also if you wait too long the app/alarm disconnects (25 seconds, idle time-out?), the app reconnects after 5 seconds but I was no longer able to read. So it seems that after a first failed try to read all subsequent tries will fail too.
For now I am able to consistently get the app to work by doing a reinstall, configure and save settings, wait 5 seconds and then click read.