Multi-page scores yield invalid MusicXML
As far as I can tell, any score of more than one page yields an invalid MusicXML when exported. In particular, there is a validation error regarding the system-distance tag at the top of the second page (either that or the error is spurious; the score does actually load if you ignore it).
Simple steps to reproduce:
1) default empty score
2) add page break after measure 4
3) export to MusicXML
4) try to import
Result: "Fatal error: line 125 column 27 Element system-distance is not defined in this scope."
If you go to Edit / Preferences / Export and turn off "Export all layouts", there is no error. Any of the other settings seem to be good.