There is probably a command within your profiles that is also using the shift key, and there may be situations where the key ends up in a constantly-pressed state. Although VA is smart enough to know the difference between keys that it is, 'pressing' and keys that the user are pressing, there could be some kind of conflict there when it's all happening at the same time. I would try switching to a less commonly used keyboard key like F1, F2, F3, etc., or a key that you know is not being used by Elite to do your PTT.
In regards to the error, you'll want to look in your VoiceAttack installation directory and see if there is a file called, 'VoiceAttackFault.txt'. If you find that file and the date is recent, you'll want to post it here so folks can help you fix your system so that VA will run. If you do not have that file available, you'll want to try disabling plugins as it's likely you've got something up with one of them. To do that, go to Options > General > Plugin manager and then deselect all the plugins. Try adding them back one by one to see if one is the culprit.
Hope that helps!