The error you are describing was a bug in some of our older template files from when we moved our tool set from 4.4 to 5.0. Our latest plugin should prompt you to update your InDesign file when you open it if that InDesign file contains content that needs updating from a 4.4 file to a 5.0 file. "Invalid Object", previously referred to Popup Objects not being converted properly.
Can you try reinstalling the tool set but first deleting all of your Template files found in the "Mag+" folder in your Applications. Then try opening and running the Multi-Device Export tool on one of your files.
Mag+ Support Team