I had this problem recently on a V6 box I have had for years but never used but then wanted to use. Error 7400 means the box needs activating but to do this on an old box you need to speak to technical support. Technical support weren't able to activate it either and said the box had a fault and it needed to be replaced.
You're best to call technical support. The queues in the daytime can be long but if you can call at 8am when they open you'll get through quickly.