Hi Ross,
I was having a similar issue (i.e. same error), but it was caused by a different problem. There maybe some overlap. I have since resolved my issue and might be able to lend a hand to yours. Is it the case that you've now create a new Fiori AppSite in your trial instance, and are trying to access it via. FioriLaunchpad.html?
Let me know and I can suggest a solution (if our problems are indeed similar).
Cheers,
Hagen