Seems bizarre that having identified it didn't seem to be a valid route, rather than simply rejecting it, the system effectively switched off.
30+ years ago when I was submitting flight plans at Blackbushe Airport, I filled them out on paper, passed them to Reg, who ran the tower (technically a FIS operator not an air traffic controller but he acted like he was), who read through them and then handed them back if I'd done them wrong.
For the interim report to state
"Clearly a better way to handle this specific logic error would be for FPRSA-R to identify and remove the message and avoid a critical exception. However, since flight data is safety critical information that is passed to ATCOs the system must be sure it is correct and could not do so in this case. It therefore stopped operating, avoiding any opportunity for incorrect data being passed to a controller"
is an abysmal admission of the failure to properly set up the automation to cover this scenario.
What's also worrying is that they couldn't easily work out which flight plan had caused the issue, which why they ran out of time for the 4 hour buffer of data and then had to switch to processing everything manually. Surely their error logs showed what the system was doing at the time of the critical exception?
Edit:
Actually, thinking about it, as parts of NATS are based on 1960's technology, it's entirely possible the error logs are extremely limited in the actual data stored because 1960 computers obviously had very little storage space.