Eurotronics Spirit thermostats do not react

I have 12 Eurotronics Spirit thermostats connected, but non of them reacts on temperature changes since 2 days. I’ve restarted the app, the full Homey and also installed an additional one (installation went fine), but after 3 minutes the same behaviour - no reaction. No error message occurs. Any idea to help?

The red errormessage is “TRANSMIT_COMPLETE_NO_ACK”

That seems an out of range issue.
It’s best practice to use the official app topic for this Q., the developer gets notified as well.

Where to find this info:
App support and/or forum links are present at the bottom of app store pages (if provided for).