Help - Afraid to press the "Flush OTAA device nonces" button

One of our sensors, upon joining flagged the error “DevNonce has already been used”.

I read in other forum articles that the “Flush OTAA device nonce” button can be used, but… does this button flush the devnonce for this one device or does it affect the system.

Just the one device, it’s essentially harmless to press the button. Really the only thing that could go wrong is if someone was trying to do a replay attack on that device and held on to a join request from before you press the reset button and tries it again after. Which frankly will never be the case unless you are a large business or they knew you were doing the reset.