When I first read these articles, I thought the concerns some people had with the testing video showing the ride's very slow movement around some inversions had become quite real and that the thing had actually become stuck upside-down with riders on it - that would have been a real problem.
This...I doubt it is anything too serious. Rides ALWAYS have technical problems in the early stages of their operation. Opening press events, if they want to take place before the ride opening, will always run the risk of going ahead in a riskier time availability-wise, but they will of course go ahead and host them anyway as the publicity is good either way. Wasn't the breakdown situation with Saw much worse?
Edit:
http://www.dailymail.co.uk/tvshowbiz/article-1161461/Dozens-stranded-new-13m-Saw-ride-breaks-Thorpe-Park-celebrity-launch-party.html