It is a well known, well documented and very long standing fault on the website.

Quite frankly, the fact that VS have never bothered to fix it reflects very badly on them, and their IT system IMHO. Having said that, there is a lot about VS IT that appears not fit for purpose

On the upside, in the error page is an option for a 'ring back' from Virgin. I used it a couple of days ago, and was called within a minute or two, and was able to book the seats I saw.

Nick