I just got this addon, and I did notice that on some airport voxatc just boots up fine and some airports it is stuck on Completing System.
Here is what I have found.
Airports with Ground tower etc - Voxatc starts up fine without having a flightplan loaded.
On airports with no Ground tower (smaller GA airports) - it needs a flightplan of sorts to have it bypass the Completing system hangup.
I do not know if this is a bug or a feature but this should help you guys