Smart Thermostate Inverted Screen with Error Code?

Hi!

I have multiple radiator valves in action and only one of them reacts sort of weird? 
It shows an inverted (white on black) ePaper display with the text "08 !".

Besides that, it seems to work fine. Batteries where changed and I had a message about calibration failure, but running a calibration through the app worked just fine.

So, what is that text "08 !" and how to get rid of it?

Thank you!