Where can I find anything on error code 400.003.303?
This error works out to:
ERROR_UNKNOWN_HOST - Request to server failed with UnknownHostException (Android) or NSURLErrorDomain (iOS).
I'd suggest starting with the user's phone and ensuring that they have network access wherever they're using this (the phone can't be in airplane mode, or not have a SIM, for example.)
Try clicking the more button (three dots) next to the token and select migrate token. The token activation will go back to their registered email and they can try scanning it in again.
If this keeps happening, I'd suggest opening a support case -- please include details about the phone (OS, OS version, manufacturer, and if it is on network or not.)
WatchGuard Customer Support