Hi Bentley…
Did you ever find a solution to this?
We are having the same issue with the Fulldome plugin. We recently updated to AE 2015, (and got the Fulldome update for 2015 compatibility) and cant open any of our projects in 2015 without the same error message you mentioned. If we create a simple project with a solid and fulldome thrown on it in 2014 it works in 2015 but our old projects with dome setups are just not working.
This is preventing us migrating to 2015