Hi Kahoot
Hopefully swapping the boxes around as suggested by japitts has resolved the problem.
The CS2400 error mainly affects BBC channels, possibly related to the BBC iPlayer app, for which a link is triggered when landing on a BBC channel. This provides the interactivity with the red button in the same way as customers get an error message when there is no internet connectivity with the box. You can force the BBC channels to play without the pop up reappearing if you hit the back button on the 360 remote when the error message appears, and then press pause and play.
In a similar vein, instead of rebooting the box have you tried using the Home button, or the Back button to exit the app(s) then wait around 30 seconds to a minute and open the app(s) again?
There may actually be a local fault even though the mini box is working OK, and it's not apparent by the diagnostics on the main box which says the connection is good. Have you used the /check-services/ function to check for problems in your area? You can also a run a test on your equipment from there. If nothing is showing you could also try the automated Service Status number 0800 561 0061. This often gives details of more local issues down to postcode level.
Again even though the diagnostic screen shows the signal quality is good it's possible that you have a loose connection. Sometimes the white coaxial cable going into the back of the 360 has worked loose, or it has a few kinks in it. Uncoupling the white coaxial and straightening and reconnecting it may resolve the issue (if you've swapped boxes this cable reconnection may have actually done the trick)
There is another possible solution, performing a factory reset by going to Settings (the cog at the end of the Home menu) > System > Factory Reset. It's actually misnamed and only a partial system reset and not a complete factory reset. You are given 2 options
Keep Recordings
Format Disk
If you do this then Choose Keep Recordings, which means you won't lose your recordings, and any planned recordings, or series links you've set up. You may have to sign into your apps such as iPlayer, Prime, Netflix and ITVX again. Also Match frame rate and Standby power consumption may go back to their default settings if you have changed them.