Error importing XLIFF file

I received several translated files for a set of courses, all in .xlf format.  All of them work fine but one.  I am getting an error that says, "Unsupported XLIFF File: The element named 'unit' is an invalid child for the Unit element."  I saw in the community posts a suggestion for validating XLIFF files, so I ran it through that online service (dev.maxprograms.com/Validation) and I got this response:  File [filename] is not valid XLIFF (whole validation response is attached).   

I am wondering if there is any way for me to manually edit the XLIFF file to correct this error so I can import it or if this has to be done in whatever translation software our service uses.  They have been insisting the file is fine, but obviously it's not...but at this point if I can fix it and import it, I would rather just get it done.  I am wondering if I can edit it in something like Notepad which typically doesn't apply formatting, without it corrupting the file...and I would have to know WHAT to edit. I opened the XLIFF file in Notepad but couldn't find the exact string of text that was called out in the error message, so I wasn't sure you to proceed. 

Maybe the only answer is to go back (again - this will be their 5th try on this file) to the translation company....and yes, I did a fresh export each time and didn't change anything in the source Storyline file after.

Thanks for any help...or just for the sympathy if no help is available.  :) 

Robert