Hey, guys - This is an old post but it came up for me on a google search, so I wanted to answer it.
Here's a fix, from another forum:
I have a new workaround which did get me past this particular problem so it will be interesting to know if it helps
anyone else that encounters this error.
I loaded the project as normal, tried to build it as normal and hit the same error - just checking that nothing miraculous had happened overnight.
I then went into the AuthorScriptHDMVSessions folder (which is inside the folder named after the project) and deleted ALL of the files contained in there. I then got Encore to build the project again ... and it worked!
It seems that deleting those files causes Encore to re-import/re-analyse all of the assets and this corrects whatever is causing the problem.
Memo to Adobe: PLEASE improve the error messages in Encore! They stink at the moment - they are almost totally useless in suggesting what the end-user can do to correct the problem.