E7 is air in flow pipe detected.
Hotter water can hold less dissolved air. It will cavitate earlier.
It’s also possible that entrained air in dhw is simply not clearing the coil ever. I can’t comment on plumbing other than to say this issue is nowhere near as prevalent in the district heading space.
The manufacturer is following the mbus standard here so had already told you everything that you need to know there.
The error values displayed are being incorrectly parsed by the mbus parser. These are NOT the same register as the regular values. Get a proper mbus parser and you’ll see that they have different names and can be parsed/filtered accordingly. 
I’d still try to get air from your system though.