Steps: Load audio wave file, from effects rack select Special > Mastering.
Audition has encountered a fatal error and needs to shut down.
This does not happen when selecting other plugins in the same tree.
I did choose the option to import presets from previous version which I suspect might be causing a conflict as I did have custom presets from Audition CS5.5 for Mastering.
For me, I'm talking about the PC version (I don't think those specific hardware options exist on Macs). All I know is that Mastering would cause Audition to crash without exception until I switched hardware modes and then I never had a problem again after that. There likely could have been a temporary hardware/driver conflict with some other software using audio channels because I have since (after several system reboots) switched back to WASAPI and do not get the plugin crash now.
We need to know what the OP's OS is in order to help him not yours I'm afraid. We already know that WASAPI and MME are Windows PC drivers.
But the problem is more likely to be caused by a difference in the Mastering Effect's settings file between CS 5.5 and CC. Perhaps Durin can spot a difference in the XML settings file but I expect that he is tied up at NAMM at the moment.
I think my first hunch at troubleshooting this would be to rename the preferences folder and relaunch Audition, so that it starts with fresh, clean, non-migrated preferences and see if the crash still occurs.
If so, then the presets and preferences are likely NOT the problem and you can rename your old preferences folder back. Then we need to do a little more hacking and find out what Audition or the OS thinks is causing the crash. This will be different depending on the OS.