Boosting issues have nothing to do with the engine coolant sensor so its not that at fault.
Your fault code indicates a boost leak
Your symptoms indicate your going into limp mode.
You are definitely going into limp mode for some reason or another. The fact an engine off, engine start again rectifies it, definitely tells you that its going into limp.
So why is it going into limp?
What fuel are you using? Does your wife fill it up or do you? You will go into limp if your filling it up with normal 95 ron UL. For full boosting on remapped you need to be feeding it 97/99. It will happily run on 95 by retarding, but to hold and keep max boost without ecu putting you into limp it needs SUL97 at very least. I tested this a couple of weeks back by putting in 95ron, my ecu wouldn't allow me to rev at max boost past 4500rpm, as soon as i reached this, ecu pulled like a mofo and then i couldn't boost past 10psi for the remainder 4500-7000rpm. Switch off, boost back, upto 4500rpm, it was fine. After that, ecu retarding and putting it in limp.
If you are running it on 97/99 then like others have suggested, dumpvalve and boostleaks should be your next check.
Standard DV on a remapped car is a no-no. They're prone to failure especially with increased boost levels. You need this changing anyway
What map is it?
Boost leaks can be detected via logging actual and requested boost with vagcom. If you know
how to use it, also log the CF values and maf values.
Warranty wont sort boosting issues with a remapped car. has nothing to do with SEAT, its you who has raised their 10psi standard level to 21psi remapped.