Error when importing TestRail test cases into Zephyr Scale

I am attempting to import test cases from TestRail (another team's test cases) into Zephyr Scale (my test case management tool). When I attempt to import the XML file, I receive the following error: 

The uploaded file could not be parsed. It seems to have invalid content.
Try again by uploading a valid file.

I can find no information on what constitutes a "valid file" and the file that I have seems to be a typical XML file. Are there instructions for what constitutes a valid XML export from TestRail, or additional information for how to import this file into Zephyr Scale? We need to import these test cases and I am blocked completely at this point (CSV import doesn't work either).

Thanks!