Error 20 is: "Unsupported command Unsupported or invalid g-code command found in block."
It might be some proprietary command Easel adds to its gcode files. I just don't know the answer.
If you can zip up a gcode file that gives the error and email as an attachment to me, I will be happy to look into it for you. Click on my "Picengraver" icon name besides this post, and you can send an email to me.
You might also ask on the Inventables' forum for help: https://discuss.inventables.com/latest