Further to previous (computer) coding errors as reporterd inter alia here:-
Latest is if you are in the agent tax account and try clicking on the 'breadcrumbs' e.g.
Agent services home > PAYE client list > Client home
Then any other than the 'Client home' option tries to resolve to a localhost address so you get a "This site can’t be reached localhost refused to connect." error message.
Obviously someone's been playing with the coding and hasn't properly tested it.