The system is down: what the law actually asks of you
A fair worry about a legal duty that lives in a government computer: what happens when the computer is down? The regulations answer it directly. Regulation 6 sets out an outage procedure, and it is stricter than most people would guess.
The deadline does not pause
An outage does not suspend your duty to make the record. You must still make a written record of all the required information within the normal deadline, the end of the second working day after the day the waste arrived. Electronic form counts, provided it is easily retrievable. You must also notify the agency of the outage as soon as reasonably practicable, unless they told you about it first.
Then seven days to catch up
Once the outage is resolved you have seven days to enter those written records into the digital system properly. Until that is done you must keep the written record and produce it if the agency asks. Failing any step of this is one of the offences the regulations create, so "the site was down" is a procedure to follow, not an excuse to rely on.
What counts as an outage
The definition covers circumstances beyond the operator's control that prevent access to the system by the deadline. Defra's service being down qualifies. Your own connectivity failing at the gate is the everyday version of the same problem, which is why software that records everything on the device and submits by itself when signal returns effectively runs regulation 6 for you: the record exists from the moment of receipt, it is retrievable, and it enters the system the moment it can.