To summarise - Running an instance of Nebula Pro on a Mac running OSX 10.6.7 and using Audiofile Engineering RAX as the host was not generating the .SER file for authorisation. Additionally, the plugin was getting stuck at the Step 8 - "program #0 not valid" error.
I attempted to remove all of the Nebula files that I could find from the following locations:
a) /Library/Audio/Presets b) /Library/Audio/Plug-Ins/Components c) /Library/Audio/Plug-Ins/VST/Acustica
Doing this and reinstalling did not solve the issue.
The problem seems to have been caused by mistakenly having copies of the VST version in two separate locations:
A version in c) /Library/Audio/Plug-Ins/VST/Acustica
and a version in:
When I attempted to remove the plugin and start again I missed the ones lurking in location d).
Removing the VST plugins from location d) as well as location c) solved the problem. The serial file is now generated and the plugin goes to the "Init" screen correctly.
Hope this is clear enough and helps anybody with the same issue.