There can be a variety of reasons for this error. Some things to check:
* Is the Aurora set to the same Sync Source and Sample Rate as it was the last time ProTools was open?
* If the Aurora is set to 192KHz, try changing it to a lower sample rate before launching ProTools.
* Sometimes Audio Midi Setup can "hijack" the Avid card, and prevent proper loading of ProTools. Try removing these two files, empty trash, and reboot:
Library/LaunchAgents/AvidAudioServer.plist and Library/LaunchDaemons/com.avid.AvidAudioServiceHelper.plist