WordprocessingML v. 2 error message in Studio 2019 (unexpected content)

Hi

Been getting this error message for two unrelated word files in the last two days. The workaround suggested here for Trados 2017 worked for one of them (unchecked this file type and used Word 2007 v. 2.0.0.0) in the new project wizard, but haven't gotten to finalizing this project yet, so who knows if it keeps. However, why am I getting this at all, for a wordx file that I just saved locally on Word 2016? What is going on?

Thanks 

Shmuel