I'm working over the sample project. I think I found a simple bug, when I select a line (for example the line below "CAMPVIVES" in the sample project) and try to export always receive an export error.
If I select something else it works.
Thank you for your report - nice catch! I will file a bug internally.
In the meantime as a workaround, you could export an SVG from a line, or recreate it with the rectangle tool and export as PNG.
Just a quick update: this has been fixed internally, and the fix should be available in the next preview release.