Hello,
I am using a Qubino Flush OnOff Thermostat 2 to control my solar thermal warm water heating system.
Generally it works fine, but always the (water) temperature exceeds 80.6 °C the value shown on ZWave-JS is -888.8 °C.
I have no other device so that I can not check if the Qubino is out of order.
Has anybody the same problem or can anyone help me?
Thanks a lot,
Marco
Did you contact already the customer support of Qubino? With the price tag they have they should have superior support over other brands selling comparable hardware for (much) less. 
Actually Qubino even advertises on their website: 
100% Customer Support
Our support policy advises that every customer receives an answer within 24 hours.
Go to our support website and open a new support ticket. By telling us the product’s unique serial number and part number, we will automatically review the production log file containing device parameters and information. This allows us to immediately identify and address issues, giving you the best customer support.
With all that marketing bla bla I wasn’t able to find specs of devices they sell on their website - do you know the temperature range of the probe they sold you? Maybe it’s just end of measurement and -888.8 is the value your Qubino shows when out of range? 
Thanks for bringing the solution:
The range of this Qubino is in fact -25 °C - 80 °C. I never would have thought that and I was sure that it’s up to 100 °C, but you can read it in the manual.
My offset is 0.6 °C so it works a little bit longer in my case.
Thanks again, Marco
Quite poor range for such a pricey product. Our hot water solar tank can easily reach over 90°C in summer time 
Luckily our thermostat based around a cheap DS18B20 temperature probe, a relay and a esp (in total less than $15) doesn’t has such shortcomings. Powered by esphome it allows full local control and ownership 