Export to sibelius broken - "This is not a MusicXML file and cannot be loaded: Element bass is not defined in this scope.

Tried that too and it still didn't work. I have been struggling with this for hours today. I began trying to import a .mxl file (compressed) that I downloaded from the Musescore website. I got the "This is not a MusicXML file and cannot be loaded" error message. So, I tried opening the M usescore file and exporting from there as a .xml file and as a .musicxml file. Both of those resulted in the same error message. I then tried simply using the "open" function in Sibelius with all three file types. Same result. Has anybody found a solution to this issue?