File will not save as docx. only doc.

[Library Worker]

A student is attempting to save a document in Word 2016. She selects the save destination, names the document, and hits save. The file appears for a second in the destination (e.g. "desktop" or any number of the devices we've tried) and then quickly disappears. When it disappears the "save as" prompt pops back up. No error notice or explanation appears.

After messing with save destination, file name, and few other variables, we switched the format to doc. from docx. This time the file appeared in the destination and stayed there.

Aside from curiosity leading me to get to the bottom of this issue, the student is inconvenienced by the necessary format change, as a doc. save does not save the citations she generated in Word.

Has anyone experienced this issue before? Any pointers would be very appreciated.

thanks.