OS X Mavericks :: IMessage Not Working On One Machine

I consider myself as having a decent knowledge of Mac products and their workings, but this has got me completely stuck. I have two mac machines (MBP and MacMini), an iPhone and an iPad. iMessage was working fine on all devices up to a couple of weeks ago. On the MacMini, all of a sudden iMessage stopped receiving and sending messages, and displaying the recipient's name in red, instead of the usual blue (clicking on the name, it says the user isn't registered with iMessage, and it says that for all users). It still works fine on all of the other three devices. 

I have tried everything that I've found on the forums - resetting the iCloud password, the keychain, the pram, deleting various files from the user library folder. Logging out on other devices, even a clean installation of 10.9.3 (which is also on the other, working machine). There is no firewall, and another exact same model MacMini that my colleague uses over the same connection works absolutely fine! 

It seems that the problem is almost tied in directly to my specific machine - is this possible? as I've already spent far too long trying to fix this.