I've seen a similar error in the past with Project Maestro (Tableau Prep beta version).
Are you using a beta version?
If "yes" can you please download and install latest Tableau Prep version from the link below and test if the issue still persist.
Hope this help.
I'm using "Tableau Prep 2018.2.1" I believe this is the latest version.
I do not have this problem when using the data interpreter with the same files in Tableau (here I have the version "Tableau 2018.1".